Fluctuations in plasma testosterone levels in adult male rats and mice

The effect of chronic administration of a synthetic LH-RH analogue intranasally in cryptorchid boys

Eighteen boys with either unilateral or bilateral cryptorchidism were treated with a synthetic LH-RH analogue ("D-Leu 6, Des-Gly-10 LH-RH ethylamide") intranasally. The peptide was dissolved in aqueous solution (25 micrograms in 0,2 ml) and administered in the form of nasal drops. The patients were divided in two groups: in group A, 50 micrograms of the synthetic LH-RH analogue were administered intranasally every 48 h for 36 days; in group B same dose was given every 24 h. Additionally, in 4 cases a LH-RH test prior the trial was performed with the same peptide. The nasal administration resulted in a fivefold increase of LH and of FSH plasma concentration in 30 min and in 60 min, respectively. The endocrine profiles for T, LH and FSH were studied in each group over the treatment period and in group B (same dose was given every 24 h) a significant decrease of the LH and FSH plasma levels could be found. The clinical effect of treatment was same in the both groups. In the whole material 44% had either unilateral or bilateral descent of the testis after the therapy.

Testosterone levels in the male laboratory rat: variation under experimental conditions

This report describes the variation of testosterone (T) levels in male Wistar rats (250-300 g) bled under typical laboratory conditions. Plasma T in 114 rats ranged from 2-48 nmol/l (median 13.1; 95% confidence range, 10-15) and varied significantly (P < 0.01) during the day (07.30-16.30 h) with an acrophase between 09.00 h and 13.00 h. The distribution of plasma T levels was skewed (P < 0.05) and could be divided into basal values < 26 nmol/l and peaks > 26 nmol/l. Testosterone in testicular tissue and testis veins also varied during the day and was lower (P < 0.02) between 14.30 h and 16.30 h than from 11.30-13.30 h. It was shown that plasma T may differ significantly (P < 0.02) between groups (n = 6-7) of untreated rats bled under identical conditions and it is suggested that gonadotrophin stimulation be included in experimental designs to reduce the difficulty of differentiating between normal variability and treatment effects.

Testicular torsion: late results with special regard to fertility and endocrine function

Late results were determined for 42 patients who had undergone detorsion and fixation for unilateral testicular torsion in the prepubertal and pubertal age. Exocrine and endocrine function for the testes was determined in 30 patients who had reached postpuberal age. Patients who underwent detorsion and fixation 8 hours or less after the onset of symptoms had normal-sized testicles and only slight changes in testicular morphology. When treatment was delayed and detorsion was done more than 8 hours later a marked decrease was observed in testicular size. The exocrine function in patients with torsion was reduced. The semen quality, as judged by 2 semen analyses, was normal in 15 patients, doubtful in 3 and pathological in 12. Even when detorsion was done 4 hours or less after the onset of symptoms the exocrine function of the testes was normal in only 50 per cent of the cases. In patients with doubtful and pathological sperm analyses higher follicle-stimulating and luteinizing hormone levels were observed.

Direct inhibition of testicular function by gonadotropin-releasing hormone: mediation by specific gonadotropin-releasing hormone receptors in interstitial cells

Agonist analogs of gonadotropin-releasing hormone (GnRH) have been shown to exert antigonadal effects in male and female animals. In hypophysectomized male rats treated with follicle-stimulating hormone, administration of a potent GnRH agonist caused depletion of luteinizing hormone and prolactin receptors and marked suppression of serum testosterone levels. The possibility that such direct effects of GnRH agonists on testicular function could be expressed through specific receptors located in the interstitial cells of the testis was supported by the selective concentration of a 125I-labeled GnRH agonist by the testis in vivo. Specific receptors for the releasing hormone were demonstrated in testis particles and dispersed interstitial cells by direct binding analysis with the 125I-labeled GnRH agonist. The binding affinity (Ka = G X 10(9) M-1) and peptide specificity of the testicular GnRH binding sites were similar to those of anterior pituitary and ovarian GnRH receptors. The presence of GnRH receptors in the testis indicates that these sites mediate the direct inhibitory actions of GnRH agonists upon testicular endocrine function.

The effect of prolactin on androgen response to human chorionic gonadotropin in normal men

Testicular androgen responses to human chorionic gonadotropin (hCG) were compared in normal males before and after suppression of prolactin (PRL) secretion with bromocriptine. Baseline follicle-stimulating hormone, luteinizing hormone, and PRL levels were suppressed by bromocriptine, 2.5 mg daily (P < 0.05). Serum testosterone and dihydrotestosterone (DHT) levels were reliably increased by one intramuscular injection of hCG (P < 0.05). Although testosterone responses to hCG were not significantly different in normal PRL and suppressed PRL cycles (P > 0.05), the DHT response was significantly increased in the suppressed cycle (P < 0.05), suggesting a physiologic 5 alpha-reductase blockage by PRL in men.

Androgen substitution with testosterone containing nasal drops

Testosterone proprionate in form of eye drops will not be absorbed. A good absorption takes place following the administration of testosterone containing nasal drops in the form of an emulsion of pure testosterone. A quick, significant testosterone increase but of only short duration takes place. Unpleasant side effects were not observed. This treatment may be clinically applicable to male patients in the climacteric period.

Mechanism of induction of luteal phase defects by danazol

The effects of danazol on the luteal phase of cycling rhesus monkeys and on the ability of the intact rhesus monkey to respond to hCG stimulation were studied in two experiments. In a third experiment the impact of danazol upon the response of the decapsulated mouse testis to hCG stimulation was evaluated. These experiments demonstrated that danazol shortens the luteal phase and decreases progesterone production in intact monkeys via a direct effect on the gonad.

Luteinising hormone-releasing and anti-fertility properties of a glucagon-selective somatostatin analogue

The hypothalamic tetradecapeptide, somatostatin (SRIF), inhibits the secretion of growth hormone (GH) and numerous other hormones, including insulin and glucagon. Attempts to use SRIF as an adjunct in the treatment of diabetes mellitus met with limited success due to its short biological half-life and the undesirable diabetogenic activity of its insulin-lowering properties. Efforts at synthesis have yielded SRIF derivatives with prolonged GH-lowering activity which did not suppress glucagon or had equivalent insulin-inhibiting activity as well as several short-acting compounds with the appropriate glucagon specificity. A dodecapeptide analogue [des-Ala, Gly] His-D-Trp-SRIF (Wy-41, 747) has been identified that combines selective inhibition of GH and glucagon release with prolonged activity. However, in routine pharmacological tests chronic treatment of mature rats with Wy-41, 747 produced anti-reproductive effects resembling those described for luteinising hormone (LH)-releasing hormone (RH) and its agonists. We report here that Wy-41, 747, unlike SRIF and other of its analogues tested, releases LH, induces ovulation and inhibits pregnancy when administered before or after implantation; these properties are traditionally associated with the separate LH-releasing class of peptides.

Characterization of prostatic carcinoma among blacks: a comparison between a low-incidence area, Ibadan, Nigeria, and a high-incidence area, Washington, DC

A comparison of carcinoma of the prostate gland in a low-incidence Nigerian (Ibadan) population and a high-incidence U.S. Afro-American (Washington, D.C.) population was the purpose for this study, initiated in 1973. The frequency of carcinoma (micro- and invasive) was determined in consecutive necropsy cases from hospitals in Ibadan, Nigeria, Accra, Ghana and Washington, D.C. The results of these clinical, epidemiologic and morphologic studies are reported. Clinically, the peak incidence of carcinoma of the prostate in Nigerian males in Ibadan and in American males in Washington, D.C. was in the 65-74 age group. The median age of patients was 66.4 years in Ibadan and 69.2 years in Washington, D.C. Seventy-five percent of Nigerian and 49% of American patients were in stages III and IV. Overall, the distribution by grade of neoplasms in surgical material from Ibadan and Washington, D.C. was similar. Plasma testosterone and estrone levels were significantly higher (p < 0.05 and p < 0.001) in U.S. patients than in U.S. controls and higher (p < 0.001 and p < 0.05) in U.S. patients than in Nigerian patients with prostatic carcinoma. Plasma testosterone levels were significantly lower (p < 0.05) in Nigerian patients than in Nigerian controls. Estrone levels were not significantly different in Nigerian patients than controls. Nigerian patients were more sexually active throughout their lives than American patients; however, they reported a higher incidence of impotence than U.S. patients in the immediate 5 years preceding the diagnosis of prostatic carcinoma. Nigerian respondents (patients and controls) arrived at puberty later than their American counterparts. Within each of the two population groups, the factor with the highest relative risk ratio was lower urinary tract symtoms occurring 10 years or longer before the onset of the present complaints which led to the diagnosis of prostatic carcinoma. Nigerian and U.S. patients were more sexually active than controls. Microcarcinoma (incidental, latent) of the prostate gland occurred with a frequency of 11.8% and at an age-adjusted incidence rate (world standard) of 40.6 per 1000 necropsies in the sampled U.S. black male population. The age-adjusted incidence rate (world standard) for microcarcinoma in the combined West African (Accra and Ibadan) series (36.7/1000) was almost equal to the rate (40.6/1000) in the Washington, D.C. series.

Release of luteinizing hormone in male mice during exposure to females: habituation of the response

Male mice release luteinizing hormone when exposed for a short time to a female. In this experiment, multiple blood samples were withdrawn by atrial cannulas from tethered males during either continuous or intermittent exposure to nonreceptive females. After an immediate, transient release of luteinizing hormone, continuous exposure to the same female was accompanied by only random, spontaneous elevations in plasma levels of this hormone. Successive presentations of the same female at 2-hour intervals elicited gradually diminishing luteinizing hormone responses. Exposing such unresponsive males to novel, diestrous females, however, dramatically stimulated their release of the hormone. These results demonstrate habituation of a socially induced, neuroendocrine response involving reproductive hormones.

Light microscopic stereological analysis of the normal human prostate and of benign prostatic hyperplasia

Normal prostate glands from 6 men less than 30 years old and enucleated tissue of benign prostatic hyperplasia were analyzed by stereological methods. Studies on the relative volumetric amount of fibromuscular (stromal) and glandular areas of the gland reveal no statistically significant difference between the inner and outer parts of the normal prostate and between the inner part of the normal prostate and benign prostatic hyperplasia. However, in benign prostatic hyperplasia there is a significant increase in the volumetric amount of the fibromuscular tissue and a decrease in the glandular area compared to the outer part of the normal prostate and the whole normal prostate (sum of the inner and the outer parts). These stereological data are discussed with respect to the pathogenesis of benign prostatic hyperplasia.

Electron microscopic stereological analysis of the normal human prostate and of benign prostatic hyperplasia

Normal and benign hyperplastic prostatic tissue was studied by quantitative electron microscopic measurements. Quantitative morphological procedures provide values for volume, surface, number of tissue and cellular components within human prostatic tissue. When a comparison is made of the stereological data of the glandular cell of benign prostatic hyperplasia to that of the normal human prostate no statistically significant difference in the relative volumetric amount of the cell organelles is indicated. An attempt was made to characterize the fine structure of the smooth muscle cells of the stromal area (fibromuscular) in normal and benign hyperplastic prostatic tissue. In benign prostatic hyperplasia a significant increase in the relative amount of organelles in the smooth muscle cell was found, indicating an activation of these cells. Light microscopic analysis has revealed that benign prostatic hyperplasia is primarily a stromal disease.

Perinatal exposure to cannabinoids alters male reproductive function in mice

Oral administration of delta 9-tetrahydrocannabinol or cannabinol to female mice late in pregnancy and during early lactation alters body weight regulation and pituitary-gonadal function and suppresses adult copulatory activity in their male offspring. These findings suggest that both psychoactive and nonpsychoactive constituents of marihuana can affect the development of male reproductive functions in mice.

Evidence for Leydig cell dysfunction in rats with seminiferous tubule damage

To study the effects of seminiferous tubule damage on Leydig cell function and morphology, rats were treated by fetal irradiation (to induce Sertoli cell-only syndrome, SCO), 3 months administration of hydroxyurea (HU), or chronic feeding of a vitamin A-deficient diet (VAD). Leydig cell function was assessed by the measurement of serum LH and testosterone and the response of serum testosterone to hCG stimulation, while morphology was studied by electron microscopy after perfusion fixation. Serum LH was significantly elevated in each experimental group, while basal serum testosterone was significantly lower only in SCO rats. In all treatment groups, the serum testosterone response to hCG was significantly decreased when measureed as the area under the response curve. Despite a decreased response to hCG, the Leydig cells were larger than normal and showed striking increases in quantities of smooth endoplasmic reticulum, mitochondria and Golgi complex. Leydig cell dysfunction has been demonstrated in animals with varying degrees of seminiferous tubule damage, but paradoxically the cytological features of the Leydig cells were indicative of hypertrophy.

In vivo patterns of circulating steroids in adult male rats. IV. Evidence for rapid oscillations in testosterone in normal and totally parenterally nourished animals

In vivo patterns of circulating testosterone (T) were investigated in stock fed controls and parenterally nourished (TPN) rats. Rats were sampled at 2 minute intervals for 30 minutes via a jugular cannula. Both groups exhibited a rapid oscillatory T pattern. In the control group, T concentrations at any specific time interval exhibited large differences with coefficients of variation (17-88%). In TPN rats this variation was 34-79%. Moreover, the mean T concentration of all samples obtained during the 30 minute period for each individual animal ranged from 1.3 +/- 0.1 (S.E.M.) to 3.5 +/- 0.3 (S.E.M.) ng/ml in controls and 1.3 +/- 0.1 (S.E.M.) to 2.0 +/- 0.2 (S.E.M.)ng/ml in controls and 1.3 +/- 0.1 (S.E.M.) to 2.0 +/- 0.2 (S.E.M.) ng/ml for TNP rats respectively. The mean coefficient of variation in control animals, however, was twice that of TPN rats, indicating that variation in basal T may be minimized during intravenous feeding. The occurrence of rapid oscillations in T of both stock and parenterally fed animals shows that nutritional regimen does not affect this phenomenon.

Concentrations of circulating steroids in normal prepubertal and adult male and female humans, chimpanzees, rhesus monkeys, rats, mice, and hamsters: a literature survey

Radioimmunoassay (RIA) data on concentrations of circulating steroids in normal prepubertal and adult male and female humans, chimpanzees, rhesus monkeys, rats, mice, and hamsters have been collated from the literature. Few reports include data for both sexes, for age groups, or for more than one species. In selecting references for inclusion in the tables, efforts were made to choose data only from RIA procedures that were adequately validated. A number of similarities can be found by reviewing the tables. Levels of estradiol appear somewhat similar for humans, chimpanzees, and rhesus monkeys of both sexes. Among the notable differences are the levels of estradiol and progesterone in primates and rodents, the apparently high level of aldosterone in mice, and the patterns of progesterone secretion in mice and rats. All values in the tables have been converted to picograms for easy comparison between steroids and species. Data for humans are fairly complete, but there is a significant lack of information for several other species.

Serum LH and FSH following passive immunization against circulating testosterone in the intact male rat and in orchidectomized rats bearing subcutaneous silastic implants of testosterone

Subcutaneous Silastic implants were designed to release quantities of testosterone approximating that produced by the rat testis and appropriate testosterone treatment was found to produce a physiologic inhibition of both LH and FSH secretion in orchidectomized rats. In rats bearing such testosterone implants, intravenous injection of an ovine anti-testosterone serum (0.3 cc) was sufficient to completely abolish the inhibitory effects of the testosterone implant for a period of 3 days as judged by the development of typical post-castration increases in serum LH and FSH. When this dose of antiserum was administered to intact adult male rats, the increases in serum LH on days 1, 2, and 3 post-injection were approximately 19%, 15%, and 11% of those observed in untreated castrates. Increases in serum FSH in antiserum injected rats followed a pattern similar to that for serum LH but the response was slightly greater (19%, 23%, and 22% of the castrate response) and there was no indication of a decreasing effect during the three days following injection. In these experiments, passive immunization against testosterone in intact male rats produced a clearcut stimulation of both LH and FSH release, but produced no data to support the suggestion that the intact testis can secrete nonsteroidal compounds capable of producing a differential inhibition of FSH secretion.

Plasma levels of testosterone and male sexual behaviour in strains of the house mouse (Mus musculus)

This paper investigates whether differences in measures of sexual behaviour between strains of the house mouse (Mus musculus) are related to differences in the levels of plasma testosterone. There was found to be a significant negative correlation between measures of sexual behaviour and plasma testosterone. This correlation was not behaviourally mediated, and could be demonstrated for some of the behavioural measures in an F2 generation.

Dominant-subordinate relationships in castrated male mice bearing testosterone implants

This study tested the hypothesis that subordination in male mice is directly related to reduced levels of circulating testosterone. Several measures of aggressiveness and submissiveness were quantified during or after daily encounters between pairs of intact males, castrated males, or castrated males bearing testosterone implants that were designed to maintain plasma testosterone titers at levels slightly above those of intact males. Behavioral measures included the total amount of fighting that resulted from daily pairing, the tendency for subordinates to fight back, and the relative frequency of urine marking. In all these regards, pairs of testosterone-implanted, castrated males formed dominant-subordinate relationships that involved apparently normal submissive behavior (compared to intact control pairs). Thus we found no evidence that submissive behavior was directly related to a reduction in circulating testosterone titers.

Effects of testosterone and dihydrotestosterone on spermatogenesis, rete testis fluid, and peripheral androgen levels in hypophysectomized rats

To compare the effects of testosterone (T) and dihydrotestosterone (DHT) on the maintenance and the restoration of spermatogenesis, hypophysectomized (APX) rats were treated daily for 35 days with 0.5 mg of T propionate (TP) or DHT propionate (DHTP) beginning 5 or 33 days after hypophysectomy. In the maintenance experiment, the weights of the testes and the number of early spermatids were significantly lower in DHTP-than in TP-treated animals, while late spermatids were present only in rats treated with TP. In the restoration experiment, TP increased testicular weight and the number of germinal cells, whereas DHTP had very little effect on the testis. In an attempt to explain these findings, we measured androgen levels in the rete testis fluid (RTF) and peripheral plasma of APX rats treated with TP or DHTP. The concentration of T in the RTF of TP-treated rats was nearly 3-fold higher than the level of DHT in the RTF of animals given DHTP. Plasma T levels measured 1/2, 2, 4, and 24 hours after the last of three daily injections of TP were considerably higher than were the corresponding plasma DHT levels in animals given DHTP. In animals treated with free steroids, peripheral androgen levels between 1/2 and 4 hours after the last injection were much higher in rats given T than in those given DHT, but thereafter this difference disappeared. We conclude that the difference in the ability of subcutaneously injected TP and DHTP to maintain and to restore spermatogenesis in APX rats was due to a difference in androgen levels in the testes of these animals.

Experimental cryptorchidism in adult male rats: histological and hormonal sequelae

Mature male rats were rendered cryptorchid and followed for up to nine weeks during which serial blood specimens were obtained for multiple hormonal analyses; serial testicular samples were obtained as well. In contrast with control animals, cryptorchid rats showed transient rises in FSH which returned to normal at the end of the study; estrogen levels were high in the final weeks. Plasma testosterone levels were unchanged and LH levels changed little. Light and electron microscopic studies of testicular biopsy specimens showed prompt disruption of spermatogenesis which became more extensive with time. Sertoli cells underwent multiple changes including increased lipid storage and phagocytosis of spermatozoa. Comparisons are made between the sequence of hormonal changes and that of histological changes. In addition, questions are raised concerning the ways in which Sertoli cells are affected by experimental cryptorchidism.

Cannabinoids inhibit testosterone secretion by mouse testes in vitro

Addition of delta-9-tetrahydrocannabinol or cannabinol to an incubation medium containing decapsulated mouse testes caused a significant reduction in the accumulation of testosterone in the medium. This result suggests that the reported effects of cannabis on male sexual and reproductive function may result from direct inhibition of testicular steroidogenesis by both psychoactive and nonpsychoactive constituents of marihuana.

Testosterone and persistence in mice

In view of previous studies showing that testosterone increases persistence of food searching in chicks, a single factorially-designed experiment was carried out to investigate whether a similar phenomenon occurs in male mice. Using a runway test, it was found that testosterone, injected into castrated mice, did increase persistence. It was also shown that intact males resembled more the testosterone-injected than control-injected castrates, and that females resembled neither intact males nor either group of castrates. A larger number of training trials was found to affect feeding latencies in a similar way to testosterone. Comparison of two strains differing in emotional reactivity (BALB/c and Porton) showed differences consistent with their reactivity levels.

Simultaneous radioimmunoassay of tesosterone, dihydrotestosterone, 5 alpha-androstane-3alpha, 17beta-diol and 5alpha-androstane-3beta, 17beta-diol in the plasms of adult male rats

A single thin layer chromatography and three antibodies were used for the specific radioimmunoassay of four androgens in pooled rat plasma (Sprague-Dawley adult males). The following values were found (pg/ml +/- SD). Testosterone: 3, 138 +/-173; dihydrotestosterone: 374 +/-20; 5alpha-androstane-3alpha, 17beta-diol: 284+/-24; 5alpha-androstane-3beta, 17beta-diol: 223+/-11.

Relative importance of 5alpha reduction for the androgenic and LH-inhibiting activities of delta-4-3-ketosteroids

The significance of 5alpha reduction of c19, delta1,4-3-ketosteroids in regulating growth of the rat ventral prostate (VP) was examined. The androgenic and LH-inhibiting activities of a C19 delta1,4-3-detosteroid which does not undergo appreciable 5alpha reduction were compared with those of its 5alpha reduced analogue and those of testosterone (T). In intact rats M (17beta-hydroxy-17alpha-methyl-androsta-1:4-dien-3-one) caused a suppression of VP weights and plasma testosterone concentrations, and in castrated rats suppressed plasma LH concentrations. M was considerably less androgenic and moderately less potent as an inhibitor of LH secretion than either T or the 5alpha reduced analogue of M [17beta-hydroxy-17alpha-methyl-5alpha-androst-1-ene-3-one; (5alphaM)]. 5alphaM was found to be at least as androgenic and as active as an inhibitor of LH as T, suggesting that the weak activity of M may be attributable to a lack of reduction to 5alphaM. Following incubation of 3H-M with VP minces, over 96% of the radioactivity recovered corresponded with M by TLC. Under identical conditions 32-48% of the radioactivity recovered from incubations with 14C-T corresponded with 5alpha reduced metabolites of T. This study demonstrates the importance of 5alpha reduction for both the androgenic and LH-inhibiting activities of delta4-3-ketosteroids.

Hypothalamic hamartoma: a source of luteinizing-hormone-releasing factor in precocious puberty

The presence of a hypothalamic hamartoma and precocious puberty in a 19-month-old boy provided an opportunity to study their relation. Excised tissue had the ultrastructural characteristics of an independent neuroendocrine unit -- i.e., neurons containing neurosecretory granules and blood vessels with fenestrated endothelium and double basement membranes. Immunofluorescence studies using specific antibody to luteinizing-hormone-releasing factor showed antigenicity to the factor in the hamartoma. The testicular-hypothalamic-pituitary axis was tested. Clomiphene unresponsiveness suggested a lack of maturation of central-nervous-system events characteristic of normal puberty. The negative feedback system between gonad and brain was intact but partially resistant to steroid suppression. These studies suggest that hypothalamic hamartomas may cause precocious puberty by autonomous production and release of luteinizing-hormone-releasing factor into vessels that communicate with the pituitary portal blood system.

Hypothalamic control of the mammalian sexual maturation

The attainment of sexual maturity is a complex process which requires maturation and interaction not only of gonads and reproductive tract but also of the pituitary and essentially of the neuroendocrine mechanisms which ultimately control gonadotropin secretion. One of the more attractive hypotheses of the sexual maturation presumes the existence of the sensitivity threshold of the regulating system to the negative feedback signal, which differentiates immaturity from maturity. As the subject matures this declines. In an attempt to examine further the maturational alterations of the male hypothalamo-pituitary-gonadal axis, investigations were carried out with regard to simultaneous changes of blood LH, FSH, testosterone and dihydrotestosterone levels during the course of 24 hr; augmentation of pituitary and blood LH and FSH concentrations under the stimulus of LH-RH as a function of time and age; synthesis and release of LH and FSH in the testosterone-blocked animals at various stages of sexual maturation; and in vitro biotransformation of testosterone to its 5 alpha-reduced metabolites by the pituitaries as a function of age. Evidence from the experimental data could be interpreted as a decrement of the feedback set-points during sexual maturatio as reflected by the transition of the responses obtained under various experimental signals. In parallel to these observations, new evidence was presented regarding not only quantitative but qualitative changes in the pituitary gonadotropins as response to the negative and positive feedback signals. This leads to new thinking with regard to the hypothesis of differential sensitivity.

Patterns of plasma free fatty acid concentrations in fasted, resting dogs

Free fatty acid (FFA) concentrations were determined in arterial plasma samples obtained during short or prolonged fasts from resting, conscious adult male mongrel dogs. Arterial blood was continuously collected during experiments ranging from 2.25 to 15 h, from dogs fasted 1 or 5 days. Each continuous collection stream was divided at 45 equal intervals into separate sequential samples. In 18 experiments, no consistent pattern was found in plasma FFA concentrations: the concentrations were never constant, and fluctuations were only rarely periodic. Large, spontaneous, seemingly random level changes with amplitudes of 200-600 mueq/liter were observed in 14 experiments, that took from 1 to 8 h to complete. There was no apparent relation between these fluctuations and time of day or rectal temperature, and glucose concentrations remained unchanged during the FFA fluctuations. The inconstancy of plasma FFA levels and the irregularity of their fluctuations suggests that FFA production rates may be unregulated or only loosely regulated within a wide regulation band during the first few days of starvation. No current model of FFA metabolism preducts the observed patterns of FFA levels.