Conditioned taste aversion to injected flavor: differential effect of anesthesia on the formation of the gustatory trace and on its association with poisoning in rats

Anesthesia disrupts formation of conditioned taste aversion (CTA) when induced before presentation of the gustatory stimulus but does not prevent association of the already formed gustatory trace with delayed poisoning. The transition between the disruption-prone and disruption-resistant phases of CTA acquisition was examined under conditions eliminating the confounding effect of anesthesia on ingestive behavior. Intraperitoneal injection of 2% saccharin (1% b.wt.) was used as an intravascular gustatory conditioned stimulus (CS) followed 2 h later by the visceral unconditioned stimulus (US) (LiCl 0.15 mol/l, 2% b.wt.). Pentobarbital anesthesia (50 mg/kg) prevented CTA formation when applied 4 h before to 30 min after saccharin injection, but was ineffective in the second half of the CS-US interval (1-2 h after saccharin administration). CTA acquisition was also impaired by subanesthetic dosages of pentobarbital (10 and 20 mg/kg) preceding i.p. injection of saccharin. It is concluded that the abrupt disappearance of the disruptive effect of pentobarbital in the middle of the CS-US interval marks the formation of the gustatory trace which mediates CTA learning even when both CS and US are applied by i.p. injection.

Dependence on phenobarbital but not pentobarbital using drug-adulterated food

A drug-adulterated food (DAF) method was used in an attempt to establish physical dependence on either pentobarbital or phenobarbital in male CF1 mice. The mice were acclimated to the control diet for four days and then assigned to treatment groups. Group I continued to receive the control diet; Group II received pentobarbital, 5 mg/g food, increased by 5 mg, daily, for 14 days; Group III received pentobarbital, 5 mg/g food, increased by 5 mg, daily, for 7 days; Group IV received pentobarbital, 10 mg/g food, increased by 10 mg, daily, for 7 days; Group V received phenobarbital at 2.5 mg/g food for 5 days and then 3.0 mg/g food for 2 days. During drug administration, all mice were monitored daily for signs of intoxication, change in body weight, and food consumption. At the end of the drug exposure period all mice received control diet and were observed for signs of withdrawal. Mice in Group II and Group IV demonstrated significant declines in body weight and food consumption and an apparent increase in the degree of intoxication but not signs of withdrawal. Group III mice demonstrated little sign of impairment during exposure to pentobarbital and no withdrawal syndrome was observed. Mice presented phenobarbital were found to exhibit a significant degree of intoxication and a withdrawal syndrome was demonstrated. The data suggest, with respect to the drug administration schedules used, that the DAF method was not suitable for the establishment of physical dependence on pentobarbital in mice.

An effective combination of anaesthetics for 6-h experimentation in the golden Syrian hamster

The anaesthetics described for use in hamsters to date are suitable for the performance of short-term experimentation. However, an anaesthetic regimen was required which would provide a stable preparation for 6 h and hence, a suitable combination was developed. In the first set of experiments, the effect of anaesthetics (chloralose, urethane, and pentobarbital) were examined alone and in combination on arterial blood measurements. In the second set of experiments the effect of the combination of anaesthetics on arterial blood measurements and minute ventilation was examined for up to 6 h. Chloralose, urethane and pentobarbital when used alone in the hamster were considered inadequate for our needs. Chloralose did not produce adequate surgical anaesthesia whereas urethane and pentobarbital resulted in marked respiratory depression. Urethane also produced a trend towards metabolic acidosis. In contrast, the combination of agents resulted in surgical anaesthesia and the arterial blood measurements were adequate. Further, the use of the combination of anaesthetics in hamsters resulted in a stable preparation where arterial blood measurements and minute ventilation were maintained in a good range for up to 6 h. The combination of chloralose, urethane and sodium pentobarbital in hamsters should prove useful in long-term non-recovery experimentation which requires early surgical intervention, minimal respiratory depression and an even depth of anaesthesia.

Induction of physical dependence on pentobarbital by new infusion method in the rat

1. A new intermittent intravenous infusion method was developed for the induction of tolerance to and physical dependence on pentobarbital in rats. 2. Female and male rats were injected with pentobarbital (20 mg/kg/injection) through an implanted intravenous cannula. 3. The rats were allowed to receive an injection after the completion of a fixed amount of behavioral activity counted from the preceding injection, and therefore, the sedative and hypnotic effects of pentobarbital were used as an index in the determination of the injection intervals. 4. During pentobarbital treatment, the number of pentobarbital injections per day rapidly increased and stabilized (approximately 40: male, approximately 30: female) on the third to fifth day. 5. Upon withdrawal, the female and male rats who were maintained on pentobarbital administration of more than 30 and 40 injections/day for approximately 10 days manifested withdrawal signs which included spontaneous convulsion. 6. These results suggest that a new infusion method exists to produce a high degree of physical dependence in rats on a short-acting barbiturates, pentobarbital.

EEG power spectra in rats during compression and during pentobarbital infusion at pressure

The EEG of rats was studied with power spectrum analysis under compression to 71 ATA with He-O2 and during constant rate infusion of pentobarbital at 71 ATA and at surface. During compression the 1-4 Hz delta activity was increased, and 8-11 Hz alpha, 12-14 Hz spindle, and 16-10 Hz low beta activities were reduced compared to control animal recordings. During the course of pentobarbital infusion at 71 ATA, this picture changed: the delta activity was reduced while alpha and spindle activities (barbiturate spindles) increased, considerably more than in the rats studied at surface pressure. The findings may only in part be explained by pressure reversal of anesthesia.

Oral pentobarbital intake in rhesus monkeys: effects of drug concentration under conditions of food deprivation and satiation

Pentobarbital-reinforced behavior was studied in four rhesus monkeys. A pentobarbital solution and water were concurrently available during 3-hr sessions; water was freely available between sessions. Both pentobarbital concentration and feeding conditions (deprivation versus satiation) were varied. In two food-restricted monkeys subsequent food satiation eliminated pentobarbital-maintained responding. In two other food-restricted monkeys the effects of food satiation varied with the drug concentration. At the highest concentration, 4 mg/ml, food satiation did not alter responding, whereas at 2 mg/ml a moderate decrease occurred and at 1 mg/ml responding was greatly reduced. During the food satiation phase, when the concentration was 4 mg/ml, responding was well maintained under several fixed-ratio sizes. Large quantities of pentobarbital were consumed, and intoxication was observed. Water-maintained responding occurred at low rates and did not vary across feeding conditions or drug concentration. The results support an interpretation in terms of a behavioral mechanism of action. Specifically, the effects of food deprivation on drug self-administration are to increase the magnitude of the reinforcing effects of the drug.

Oral absorption of sodium pentobarbital and effects on gastrointestinal function

An anaesthetic dose (40 mg/kg) of sodium pentobarbital (SP) was administered intragastrically to adult, fasted male rats at total dosing volumes of 1, 2, or 3 ml. Area under the plasma concentration-time curve did not differ for the various dosing volumes. During the initial (rapid) phase of gastric emptying all solutions containing drug appeared to empty at a faster rate than distilled water (2 ml). However, solutions containing drug emptied much more slowly than water during the terminal (slow) phase of gastric emptying. Drug solutions moved along the small intestine at approximately the same rate as distilled water. SP significantly lowered rectal temperature and slowed the propagation velocity of the migrating myoelectric complex (MMC) recorded from four serosal electrodes chronically implanted along the proximal small intestine. In addition, the drug significantly decreased the rate of MMC recurrence, extended the duration of MMC phases, and decreased slow wave frequency at all sites monitored. The results suggest that: 1) the relative oral bioavailability of sodium pentobarbital is not influenced by dosing volume within the range tested, 2) the various dosing volumes of SP tested moved through the gastrointestinal tract at an equal rate, and 3) orally administered SP significantly decreases the propagation velocity and periodicity of the MMC in the small intestine.

Assessing pentobarbital preference in normal volunteers using a cumulative dosing procedure

Preference for pentobarbital was assessed in 12 normal healthy volunteers using a seven-session cumulative dosing choice procedure. On the first four sessions subjects sampled the drug and a placebo, and on the last three sessions they chose the substance they preferred. During each of the sampling sessions they ingested, at 30-min intervals, five capsules containing either pentobarbital (30 mg per dose) or placebo. During the choice sessions subjects first chose which capsules they preferred to take (drug or placebo), and then took from one to seven of these capsules, separated by 30 min between ingestions. Self-report measures of subjective effects were obtained at regular intervals during each session. Subjects chose the pentobarbital-containing capsules on average 52% of choice sessions, and ingested an average total dose of 132 mg. Although the drug produced only modest, sedative-like subjective and behavioral effects and there was little evidence of euphoric effects in the group as a whole, individual differences in drug liking and choice were observed. The results are discussed in terms of variables that affect the reinforcing effects of pentobarbital in normal volunteers, and they are contrasted to previous findings using this procedure with other sedative drugs.

IV Nembutal: safe sedation for children undergoing CT

In a prospective study of 225 consecutive pediatric patients who required sedation for CT imaging, we monitored oral and nasal air flow, transcutaneous oxygen saturation, and cardiac rate and rhythm before and after the administration of IV Nembutal. In addition, the first 50 patients in this series had blood pressures mechanically monitored at 1-min intervals. There was no significant change in the cardiac rate, rhythm, or blood pressure in any patient. Seventeen episodes (7.5%) of transient oxygen desaturation to 80% of baseline or less occurred after sedation. The patterns of oxygen desaturation in this series can be explained by the following mechanisms: (1) hyperventilation leading to hypocapnia with resultant loss of the CO2 stimulus of respiration (12 patients); (2) upper airway obstruction from pharyngeal muscle relaxation (three patients); (3) a shift in sensitivity of CNS CO2 receptors (one patient); and (4) central apnea (one patient). Oxygen desaturation normalized spontaneously in 14 patients. In two patients, oxygen saturations returned to normal after modification of head position to optimize airway patency. In one patient, mild stimulation was required to interrupt transient apnea. All but one patient in whom desaturation occurred showed oxygen desaturation within the first 5 min after IV sedation. At The Children's Hospital of Denver, IV Nembutal has been used in over 870 pediatric patients. No patient required resuscitation, intubation, or assisted ventilation. Only one patient required prolonged observation, and one patient demonstrated an idiosyncratic hyperactive response. The sedation failure rate was less than 1%. The average dose of sedation was reduced when compared with IM Nembutal because the rapid onset of activity after IV administration allowed titration of dose to patient response.

Effect of taurine on ethanol-induced sleep time in mice genetically bred for differences in ethanol sensitivity

Long Sleep (LS) and Short Sleep (SS) mice were used in this study to investigate the interaction between ethanol and taurine. Sleep time (hypnosis) was selected as an index of ethanol-induced central nervous system depression. In order to achieve a similar degree of central nervous system depression with ethanol, SS and LS mice received 5.3 and 3.0 g/kg, IP, of ethanol, respectively. When taurine (7.5, 15 and 25 mumol/kg) was administered intracerebroventricularly (ICV) to LS and SS mice immediately after regaining the righting reflex following ethanol injection, a return to sleep time was produced. This effect of taurine was immediate in onset and occurred in a dose-dependent fashion. LS mice exhibited a greater effect from taurine administration than SS mice. In another experiment LS and SS mice were given ICV TAG, a taurine antagonist (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide HCl), which significantly reduced the effect of taurine to produce a return to sleep time in the presence of ethanol. TAG did not affect ethanol-induced sleep time. In control experiments, in the absence of ethanol, neither taurine (25 mumol/kg, ICV) nor TAG (1 mumol/kg, ICV) caused a significant loss of the righting reflex (sleep time). When pentobarbital (50 mg/kg, IP) was injected instead of ethanol in the sleep time experiments, taurine (7.5, 15 and 25 mumol/kg, ICV) produced a return to sleep time in LS and SS mice that resembled the effect of taurine with ethanol in SS mice. These results indicate that taurine (ICV) can enhance the central depressant action of ethanol and pentobarbital and that the greatest effect of taurine occurred with LS mice in the presence of ethanol. It is possible that taurine may have some role in the central nervous system depressant properties of ethanol.

Effect of pentobarbital on gastric acid secretion elicited by secretagogues or electrical vagal stimulation in rats under urethane anesthesia

The effect of pentobarbital, which stimulates gastric acid secretion in urethane-anesthetized rats, on secretagogue-stimulated or electrically stimulated acid secretion was studied in the rat perfused stomach. 2-Deoxy-D-glucose (2-DG)-stimulated gastric acid secretion was markedly depressed by intravenous injection of pentobarbital, but was definitely increased by lateral cerebroventricular injection and was unaffected by injection into the fourth cerebroventricle. Bethanechol-stimulated secretion was augmented by intravenous injection of pentobarbital. In vagotomized rats, pentobarbital did not affect the bethanechol-stimulated or electrically vagally stimulated secretion. These findings indicate that pentobarbital produces both stimulatory and inhibitory effects on gastric acid secretion through a central mechanism, especially in the forebrain. It was proposed that the inhibitory response would result from some interaction of intravenous 2-DG and pentobarbital in the central regulatory system of gastric secretion.

Effect of chronic pentobarbital treatment on the development of cross-tolerance to ethanol and barbital

Recently, we reported that a chronic regimen of ethanol by intubation, which produced clear tolerance to ethanol-induced hypothermia, ataxia and sleep, produced only a marginal degree of cross-tolerance to these effects of pentobarbital. The present experiments were designed to test the reverse process by examining cross-tolerance to pentobarbital after chronic pretreatment with ethanol, chronic pentobarbital treatment by gavage conferred clear cross-tolerance to both barbital- and ethanol-induced hypothermia, ataxia and sleep. In a separate experiment, cross-tolerance to barbital- and ethanol-induced hypothermia and ataxia was demonstrated over a wide range of test doses. Determination of ethanol blood levels as well as a complete time course of absorption, distribution and elimination of ethanol suggested that pharmacokinetic alterations may play a role in the development of cross-tolerance to ethanol in pentobarbital-treated subjects. The asymmetry of cross-tolerance raises the possibility that pentobarbital and ethanol invoke tolerance by mechanisms that are not wholly identical. This possibility requires further exploration. Conceivably the actions of ethanol which mediate the measured effects form a subset of a larger range of pentobarbital actions that could provide a stronger stimulus to tolerance development.

Tolerance to ethanol and cross-tolerance to pentobarbital by isolated hearts from chronic alcoholic rats

The effects of ethanol and pentobarbital on the function of isolated working hearts from control and ethanol-fed male Long-Evans rats were studied. Hearts from ethanol-fed animals (38% of calories; 10-12 months) exhibited functional tolerance to the cardiodepressive effects of 18-71 mM ethanol in the perfusing medium. Left ventricular systolic pressure, peak relaxation rate, isovolumic pressure indexes, peak aortic flow rate, cardiac output, and stroke work of the control hearts were depressed to a greater extent than were those indexes of the alcoholic rat hearts during exposure to ethanol in vitro. Differences in the functional responses of controls and alcoholics were not the result of differences in energetics; myocardial O2 consumption, O2 supply-to-utilization ratio, and external work efficiency changed similarly in both groups of hearts during perfusion with ethanol. Cross-tolerance of the alcoholic rat hearts to the in vitro cardiodepressive effects of pentobarbital was also apparent. Peak rate of left ventricular pressure development, peak aortic flow rate, isovolumic pressure indexes, and cardiac output of the control rat hearts were significantly lower than those indexes of the alcoholic rat hearts during perfusion with 0.5 mM pentobarbital. In addition, four of the seven control hearts could not be paced during pentobarbital perfusion using 3V electrical pulses; the pacing voltage had to be raised and right ventricular pacing used to maintain stable function of those hearts at a 321 beats/min pacing rate. Myocardial O2 consumption and O2 supply-to-utilization ratios of control and alcoholic rat hearts were similar, but external work efficiency was slightly higher in the alcoholics than in controls during pentobarbital perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

An evaluation of three anaesthetic regimes in the gray short-tailed opossum (Monodelphis domestica)

The effect of several anaesthetic agents on the gray short-tailed opossum (Monodelphis domestica) was investigated. Pentobarbitone sodium at a dose of 50 mg/kg sedated the animals but did not produce analgesia or anaesthesia. A combination of ketamine hydrochloride and xylazine at 40 mg/kg and 5 mg/kg, respectively, sedated the animals, but anaesthetic levels were not attained. Halothane was most effective in producing anaesthesia in Monodelphis domestica. Hypothermia was a major side effect with all three anaesthetic regimes.

Oral self-administration of pentobarbital by rhesus monkeys: relative reinforcing effects under concurrent fixed-ratio schedules

During daily 3-hr sessions, orally delivered pentobarbital solutions and water, or two separate pentobarbital solutions, were concurrently available to rhesus monkeys according to fixed-ratio schedules of mouth contacts with a spout. First water, and then each of four "comparison-concentration" pentobarbital solutions (0.0625, 0.25, 1, and 4 mg/mL), was successively available from one spout for a block of sessions under a fixed-ratio-64 (three monkeys) or fixed-ratio-16 (one monkey) schedule. Under an identically sized fixed-ratio schedule, deliveries of a "standard-concentration" pentobarbital solution were concurrently available from a second spout. The concentration of the standard solution remained unchanged throughout testing of the series of comparison solutions. Each of three pentobarbital concentrations (4, 1, and 0.25 mg/mL) in turn served as the standard concentration. Within each pair of concurrently available solutions, the higher drug concentration maintained more behavior than the lower concentration. Thus when monkeys were provided with concurrent access to different pentobarbital concentrations, relative reinforcing effects were directly related to drug concentration. Further, the amount of behavior maintained by a particular drug concentration was dependent on the concentration of the concurrently available drug solution. Thus, the relative effectiveness of a reinforcer in maintaining behavior is a function of both the reinforcer's magnitude and the availability of alternative reinforcers in the environment.

High-dose barbiturate control of elevated intracranial pressure in patients with severe head injury

In a five-center study, 73 patients with severe head injury and elevated intracranial pressure (ICP) were randomly assigned to receive either a regimen that included high-dose pentobarbital or one that was otherwise similar but did not include pentobarbital. The results indicated a 2:1 benefit for those treated with the drug with regard to ICP control. When patients were stratified by prerandomization cardiac complications, the advantage increased to 4:1. A multiple logistic model considering treatment and selected baseline variables indicated a significant positive treatment effect of barbiturates, a significant effect of time from injury to randomization, and an interaction of treatment with cardiovascular complications. However, of 925 patients potentially eligible for randomization, only 12% met ICP randomization criteria. The results support the hypothesis that high-dose pentobarbital is an effective adjunctive therapy, but that it is indicated in only a small subset of patients with severe head injury.

Comparison between pentobarbital- and muscimol-induced feeding in satiated sheep

Twenty sheep were used to study the mechanisms by which the intracerebral administration of pentobarbital and of muscimol induces feeding in ruminants. Injections of 1 mumol calcium induced a weak feeding response at 1 h postinjection compared with control values (108 vs. 63 g, p less than 0.05). Injections of 78 mumol pentobarbital and of 100 nmol muscimol elicited strong feeding responses (p less than 0.01). A preinjection of 1 mumol calcium reduced the response to pentobarbital by about 40% but did not affect the response to muscimol. Administration of 1.1 mmol sodium chloride reduced the effect to pentobarbital by about 60% but only partially decreased the effect to muscimol. Administration of picrotoxin, a GABA antagonist, slightly decreased the feeding response to pentobarbital and to muscimol. Administration of gamma-vinyl GABA, an inhibitor of the enzyme GABA transaminase, did not affect feeding behavior of sheep at any of the doses tested (0-10 mumol). Injections of gamma-vinyl GABA followed by equimolar injections of GABA failed to provoke any feeding response. The data suggest that pentobarbital and muscimol may induce feeding by acting on a similar hypothalamic receptor complex but by different mechanisms. The lack of effect of GABA itself remains unexplained.

Aggressive physiologic monitoring of pediatric head trauma patients with elevated intracranial pressure

The management of pediatric patients with severe head trauma often requires aggressive physiologic monitoring and treatment. As intracranial pressure (ICP) increases, so does mortality. Yet attempts to decrease elevated ICP can cause physiologic and hemodynamic problems. In this retrospective analysis of 25 patients treated with ICP monitoring alone, ICP measurements plus mannitol, or ICP measurements plus pentobarbital (PB) induced coma/mannitol, survival rates were higher than predicted, particularly among the most severely injured. These results were apparently due to the intensive care and aggressive monitoring. Although barbiturates are known to decrease ICP, the adverse effects encountered, including hypotension, cardiovascular depression and arrhythmias, often outweigh the benefits. Hemodynamic abnormalities are unpredictable and monitoring with Swan-Ganz catheter is useful in managing patients with PB-induced comas. Many failures of PB therapy in pediatric patients may be related to undiagnosed decreases in cardiac output, resulting in decreased cerebral blood flow, even with well-controlled ICP.

Effects of intraseptal drug administration on pentobarbital-induced narcosis and hippocampal choline uptake

Effects of injection of drugs into the septum on pentobarbital anesthesia were investigated in the rat. Intraseptal microinjection of bicuculline (5 micrograms), arecoline (2 micrograms), and phenylephrine (5 micrograms) shortened, MK-212 (5 micrograms) prolonged, and atropine (2 micrograms) had no significant effect on the duration of pentobarbital-induced loss of righting reflex. Bicuculline and arecoline increased and MK-212 reduced hippocampal cholinergic activity as measured by change in hippocampal sodium-dependent high-affinity choline uptake after intraseptal drug injection. It is concluded that activation of the septal-hippocampal cholinergic pathway might be an important neuromechanism for recovery from pentobarbital-narcosis.

Relationship between minimum reinforcing doses and injection speed in cocaine and pentobarbital self-administration in crab-eating monkeys

The relationship between minimum reinforcing doses and injection speed was investigated by using 2 levels of speeds in experiments on self-administration of cocaine or pentobarbital in 2 crab-eating monkeys each. The experiments were conducted under a fixed ratio (FR) 1 schedule with 30-min time-out after each drug injection, wherein the drugs and saline were made available for alternate 5-day periods. The minimum reinforcing doses at each injection speed were determined by the titration procedure in which the presence or absence of reinforcing effect at a particular drug dose was judged based on comparison of the self-administration rate at that dose with the rate in the preceding saline period. The results showed that the minimum reinforcing doses of cocaine and pentobarbital tended to be higher in inverse proportion to the injection speed of the drugs.

Comparison of various anesthetic regimens in the domestic fowl

Three anesthetic agents (equithesin, metomidate, and ketamine) combined with diazepam were tested in the domestic fowl. Laparotomy and thoractomy were possible with the equithesin/diazepam combination, but only laparotomy was possible with the metomidate/diazepam combination. The combination of ketamine/diazepam did not result in the depth of anesthesia required for surgical procedures. Repeated blood pressure and heart rate measurements were recorded by use of a modified noninvasive Doppler technique. Equithesin/diazepam was our combination of choice for long-duration surgical anesthesia. The depth of anesthesia could be modulated by increasing the dose of diazepam. Metomidate/diazepam was useful when short-term anesthesia was required. In contrast to equithesin/diazepam, the metomidate/diazepam combination provided unstable anesthesia of varying depth and duration. Adverse reactions with metomidate indicated caution when using this drug in chickens; the drug also caused marked bradycardia. Ketamine/diazepam combination cannot be recommended as an anesthetic agent for use in chickens. The combination may be useful for minor surgical procedures or treatment, but not for experimental procedures that involve major surgery. Diazepam alone had a slight tranquilizing effect.

Ischemia-induced development of cerebral edema in awake and anesthetized gerbils

General anesthesia is often used to immobilize experimental animals prior to the induction of cerebral ischemia. However, anesthetics are known to alter many of the biochemical and physiological parameters used for the assessment of stroke-induced brain damage. We examined the effects of bilateral carotid artery ligations on mortality and the development of cerebral edema in unanesthetized gerbils. We found that increasing the length of the ischemic episode resulted in increased mortality, both during the ischemic period and during cerebral reperfusion. The duration of the ischemic episode was also correlated with the rate and degree of the development of cerebral edema. Both of these estimates of ischemia-induced brain damage were significantly reduced by the pretreatment of the animals with pentobarbital. Based on the variable effects of different anesthetics on CNS activities, and the observed effects of barbiturate anesthesia on ischemia-induced mortality and edema development in the present model, we suggest that it may be inappropriate to anesthetize experimental animals when investigating certain aspects of stroke-induced brain damage.

Effects of cyclosporine on anesthetic action

The effects of a single dose of cyclosporine on anesthetic actions of pentobarbital and fentanyl were studied in mice. Mice given pentobarbital 2 hr after receiving cyclosporine, 60 mg/kg, slept a statistically significant 2.3 times longer than did controls. In a second study, each of two dose levels of cyclosporine was given before each of four dose levels of fentanyl. The analgesic effect of fentanyl, measured with the abdominal constriction test, was dose-dependent. Cyclosporine significantly increased the analgesia produced by fentanyl and did so in a dose-dependent manner. Cyclosporine by itself did not produce analgesia. Plasma levels of fentanyl and binding of fentanyl by plasma proteins were unchanged by cyclosporine treatment. The results show that a single dose of cyclosporine can increase pentobarbital hypnosis and fentanyl analgesia in mice but do not establish the mechanism of these interactions.