Chronic polychlorinated biphenyls exposure on three generations of oldfield mice (Peromyscus polionotus): effects on reproduction, growth, and body residues

Effects of chronic dietary exposure to low levels of PCBs (polychlorinated biphenyls) on reproduction, growth and whole body burdens were investigated in three generations of Peromyscus polionotus. Mated pairs were maintained on a diet containing 5 mg/kg PCBs (Aroclor 1254) for 12 months, beginning exposure as young adults; matched controls received a similar diet without PCBs. Offspring were maintained on the parental regime and paired at maturity with non-siblings in the same group. In first and second generation offspring, birth and weaning weights were significantly lower in PCB-exposed animals; in the second generation, there were also significantly fewer mice born/month, longer intervals prior to birth of the first litter, and decreased survival to weaning (25 days) among exposed mice. Whole body residue of PCBs increased significantly with each generation of exposure. This study clearly shows that chronic exposure to PCBs at a dosage of 5 mg/kg depressed fertility, growth and survival in Peromyscus, and that these effects were amplified through multigenerational exposure.

Regressors of whole-carcass zinc phosphide/phosphine residues in voles: indirect evidence of low hazards to predators/scavengers

Whole-carcass residues of the rodenticide zinc phosphide (Zn3P2) and hydrolyzed phosphine (PH3) were determined for voles (Microtus spp.) that died following ingestion of a 2% Zn3P2 steam rolled oat (SRO) groats bait. Procedures involved: a three-day acceptance test to assess vole consumption (n = 27) of control SRO groats and several one-day Zn3P2- (n = 13) or control-bait (n = 4) tests to characterize onset of pharmacotoxic signs and to obtain fatally-dosed carcasses for residue analyses. Carcasses were stored in liquid nitrogen (LN2) to maximize retention of Zn3P2/PH3 residues prior to chemical determinations. Linear regressions were computed between pairs of consumption and residue variables. Main results were the following: (1) mean (+/- SD) consumption of control bait was 2.5 (+/- 0.9), 3.0 (+/- 0.9), and 2.8 (+/- 0.8) g on days 1, 2, and 3, respectively (> or = 10.6 +/- 4.6% of body weight); (2) all test-bait voles (n = 13) died approximately 4-12 h after bait presentation, with lethargy and respiratory distress key signs of toxicosis; (3) whole-carcass Zn3P2 residues averaged 1.73 mg (min-max: 0.31-4.95), and PH3 residues averaged 10.6 micrograms (min-max: 0.5-21.0); and (4) significant linear regressions were found between bait consumption/Zn3P2 intake and body weight (r2 = 0.64, p < or = 0.001), carcass Zn3P2 and bait consumption/Zn3P2 intake (r2 = 0.32, p < or = 0.043), and carcass Zn3P2 and body weight (r2 = 0.60, p < or = 0.002). Certain analytical and hazards issues are discussed.

Environmental behavior of bentazon herbicide

Bentazon is a postemergence herbicide used in early spring to early summer in many crops, usually at application rates of 1.0 kg a.i./ha. Its selectivity is based on the ability of the crop plants to metabolize bentazon quickly to 6-OH- and 8-OH-bentazon and conjugate these with sugars, while weeds do not, so that photosynthesis is disrupted and the weeds die. Also, there is a further degradation to small fragments, which subsequently are incorporated into natural plant products. Residues in the raw agricultural commodities range from 0.1 to approximately 1.0 mg/kg (straw and other "leftover" plant parts). In the upper soil layer, bentazon is quickly degraded, microbially and aerobically, via the intermediary and instable products 6-OH-, 8-OH-bentazon, and AIBA. These are immediately bound biotically and abiotically to the mostly nonbioavailable soil organic matter fraction. Additionally, a considerable part (24-50%) is mineralized to CO2. Half-lives in field soils ranged from 3 to 21 d with an average of 12 d. Abiotic degradation processes predominantly involve photolyses on plant and soil surfaces and in surface water. The physical-chemical properties and soil column laboratory studies with bentazon would seem, at first glance, to predict a leaching potential. However, several field lysimeter studies unambiguously proved that it does not leach under field situations. Annual averages in the leachates were always < 0.1 microgram/L, also after the second year. Reasons for the favorable field behavior, in contrast to the laboratory studies, are discussed. Reports on bentazon findings in groundwater and drinking water were classified as resulting predominantly from former filtrate along Rhine banks. Since 1988, the bentazon levels in the Rhine River ranged below or near 0.1 microgram/L. Very few isolated point-source contaminations, arising from accidents and other reasons, are marked by sporadic findings of concentrations > 1 microgram/L. Nonvalidated findings at various locations are reported that lie below or near the determination limit. In such situations, it is highly recommended to identify bentazon with a "full" MS-spectrum. The ecotoxicological effects of bentazon lead to "no classification necessary." The small octanol/water partition coefficient precludes bioaccumulation. Bentazon is rapidly excreted by warm-blooded animals without any uptake of residues in edible tissues. Based on its toxicological properties, bentazon was classified as noncarcinogenic ("Group E") by the EPA. The ADI is set at 0.1 mg/kg body weight/d. The WHO drinking water guideline value, based on the toxicological profile of bentazon, was recently raised to 30 micrograms/L.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of toxaphene toxicity on enzyme activity & residue levels in vital organs of guineapig

Guineapigs exposed to acute and subacute levels of toxaphene revealed a marginal reduction in the body weight. There was a significant inhibition of acetylcholinesterase and ATPases in the brain, liver and kidney. The effect of subacute toxicity of toxaphene resulted in an enhancement of cytochrome P450 and induction of aniline hydroxylase in liver and kidney. Biochemical investigations on the backbone revealed that toxaphene toxicity caused an increase in the calcium content and a decrease in the collagen content significantly. Toxaphene was accumulated more in the liver than in the kidney as reflected by residue studies.

Metabolism and excretion of dimethoate following ingestion of overtolerance peas and a bolus dose

Data to guide an exposure assessment were obtained by giving sugar peas containing overtolerance dimethoate residues (17 ppm; 8% oxon) and a bolus dose of dimethoate to a healthy adult male. The dimethoate tolerance on peas was and remains 2 ppm. Serial total urine samples were collected and analysed for dimethoate and its oxon, dimethylphosphate, dimethylphosphorothioate (DMTP) and dimethylphosphorodithioate. The dose of dimethoate administered was approx. 0.1 mg/kg body weight and produced no symptoms of toxicity. Dimethylphosphates appeared in the urine within 2 hr. The major metabolite (about 60%) was DMTP. Only traces (< 0.5%) of dimethoate and oxon were recovered from urine. Acetylcholinesterase inhibition was not observed although urinary metabolites were prominent, indicating that they are better indicators of acute exposure than cholinesterase inhibition. The results obtained using a bolus dose were virtually identical to those from the trial with overtolerance peas, and indicated that dimethoate is readily absorbed and its urinary metabolites are readily eliminated following exposures to low doses (0.1 mg/kg body weight).

Microbiological and biotechnological aspects of metabolism of carbamates and organophosphates

Several carbamate and organophosphate compounds are used to control a wide variety of insect pests, weeds, and disease-transmitting vectors. These chemicals were introduced to replace the recalcitrant and hazardous chlorinated pesticides. Although newly introduced pesticides were considered to be biodegradable, some of them are highly toxic and their residues are found in certain environments. In addition, degradation of some of the carbamates generates metabolites that are also toxic. In general, hydrolysis of the carbamate and organophosphates yields less toxic metabolites compared with the metabolites produced from oxidation. Although microorganisms capable of degrading many of these pesticides have been isolated, knowledge about the biochemical pathways and respective genes involved in the degradation is sparse. Recently, a great deal of interest in the mechanisms of biodegradation of carbamate and organophosphate compounds has been shown because (1) an efficient mineralization of the pesticides used for insect control could eliminate the problems of environmental pollution, (2) a balance between degradation and efficacy of pesticides could result in safer application and effective insect control, and (3) knowledge about the mechanisms of biodegradation could help to deal with situations leading to the generation of toxic metabolites and bioremediation of polluted environments. In addition, advances in genetic engineering and biotechnology offer great potential to exploit the degradative properties of microorganisms in order to develop bioremediation strategies and novel applications such as development of economic plants tolerant to herbicides. In this review, recent advances in the biochemical and genetic aspects of microbial degradation of carbamate and organophosphates are discussed and areas in need of further investigation identified.

Metabolism of [14C]4-chlorobiphenyl by hepatic microsomes isolated from razorbills, pigeons and rats

1. Analysis of individual PCB-isomers and congeners in extracts of adipose tissue from N = 12 razorbills suggested that 4-chlorobiphenyl was subjected to metabolism. 2. In vitro metabolism studies using [14C]-4-chlorobiphenyl as substrate showed that razorbills metabolise this substrate to [14C]4-chloro-4'-hydroxybiphenyl at an average rate of 20 pmol/mg microsomal protein/min. For comparison, the metabolism of [14C]-4-chlorobiphenyl by pigeons and rats was also studied, and average rates in the formation of [14C]-4-chloro-4'-hydroxybiphenyl of 12 pmol/mg microsomal protein min and 342 pmol/mg microsomal protein min were estimated. 3. A comparison of the hepatic drug metabolising enzyme system of razorbills and pigeons showed similar concentrations of cytochrome P-450, cytochrome b5 and comparable catalytic activities of cytochrome P-450-dependent monooxygenase, when assessed for HHDN epoxidase, PROD, EROD and the Phase II enzymes glutathiones-S-transferase, but were significantly lower, when compared with rats. The results obtained suggest fundamental differences in the catalytic activities of cytochrome P-450-dependent monooxygenase between avian and mammalian species. 4. The present study, however, provides evidence that fish-eating seabirds have the ability to metabolically dispose of certain PCB isomers and congeners, which are amongst the most ubiquitously distributed pollutants in the ecosystem.

Induction and inhibition of rat liver cytochrome(s) P-450 by an imidazole fungicide (prochloraz)

Prochloraz (1-[N-propyl-N-2(2,4,6-trichlorophenoxy) ethyl carbamoyl] imidazole) is an imidazole molecule widely used as a fungicide. This study reports the in vivo and in vitro effects of this compound on microsomal drug metabolising enzymes from rat liver. In vivo pretreatment of animals (250 mg/kg body wt for 3 days) with prochloraz elicited complex modifications. When animals were sacrificed 24 h after the last dose, an increase in total cytochrome P-450 was observed as well as an increase in catalytic activities towards benzphetamine, alkoxyresorufins and alkoxycoumarins. However, when animals were sacrificed 48 h after the last dose, a lower induction of 7-ethoxyresorufin O-deethylase and a higher induction of 7-pentoxyresorufin O-depentylase and 7-benzoxyresorufin O-debenzylase were found. Such results lead us to consider prochloraz as a "mixed inducer" of the hepatic cytochromes P-450. In vitro experiments were indicating a strong inhibition of 7-alkoxyresorufin O-dealkylase activities by prochloraz. The analysis of the CO-difference spectrum of cytochrome P-450 showed also tight binding of prochloraz to the haemoprotein in animals sacrificed 24 h but not 48 h after the last dose. Furthermore, prochloraz did not induce significantly the microsomal cytochrome P-450 IVA1-dependent 12-hydroxylation of lauric acid.

[Pesticide residues in food plants: the role of plant metabolism]

The plant metabolism of pesticides typically leads to various classes of persistent "soluble" and "insoluble" conjugates whose structures and modes of formation and storage have been characterized only in recent years. Pesticide risk assessment should include studies on animal bioavailability and toxicological properties of the conjugates.

Activities and toxicological significance of hepatic microsomal enzymes of the kestrel (Falco tinnunculus) and sparrowhawk (Accipiter nisus)

Activities of hepatic microsomal monooxygenase and epoxide hydrolase in sparrowhawks and kestrels were determined using organochlorine substrates. Monooxygenase activities were low in the kestrel and very low in the sparrowhawk compared to those in the male rat, down to environmentally realistic substrate concentrations. Epoxide hydrolase activities were very low in both species. These activities are discussed in relation to feeding ecology and susceptibility to organochlorine pollutants.

Factors in the accumulation of dieldrin in broiler organs: doses administered with feed; dose-organ-dieldrin accumulated relationship; toxicological consequences

The quantity of dieldrin accumulated in liver, kidney, heart, gizzard, lung, muscle, and intestine with contents, of broiler chickens fed with feed contaminated by this insecticide, was determined by gas chromatography analysis. The doses used were 60, 90, 120, 200 and 240 ppm. The influence of the doses used in the quantity of dieldrin accumulated in the different organs, the relationship between the doses administered, organs and quantity accumulated, and the toxicological consequences of the contamination were studied. The results show that the doses used did not significantly affect the quantity of dieldrin accumulated by the different organs. The relationship doses-organ-quantity accumulated shows that the muscle accumulates equal dieldrin at all the doses used. The differences in the dieldrin accumulated at different doses increases with the metabolic function of the organs. The principal symptoms of intoxication were anorexia, convulsions and tremors, which indicated that the nervous system is a major site of activity.

Influence of chlordecone and mirex exposure on benzo[a]pyrene metabolism of rat-liver microsomes

The metabolism of benzo[a]pyrene (BP) in hepatic microsomes isolated from rats exposed to chlordecone or mirex was compared to that of untreated rats and rats treated with 3-methylcholanthrene (3-MC) or phenobarbital (PB). Treatment with chlordecone resulted in a two- to three-fold increase in cytochrome P-450 content but the BP-hydroxylase activity per mg microsomal protein was unaffected. Addition of alpha-naphthoflavone (alpha-NF) or chlordecone caused changes in BP-hydroxylase activity indicating that chlordecone-induced cytochromes P-450 were similar to control. H.p.l.c. analyses of BP metabolites confirmed this similarity. Treatment with mirex caused a two-fold induction of cytochrome P-450, and BP-hydroxylase activity expressed per mg microsomal protein was increased 1.3-fold. Addition of chlordecone or alpha-NF caused changes in BP-hydroxylase activity, indicating differences between control and mirex-induced cytochromes P-450. H.p.l.c. analyses of BP metabolites confirmed this difference. Treatment with chlordecone or mirex increased microsomal epoxide hydrolase activity three-fold. Chlordecone accumulated in hepatic nuclei.

Aplastic anaemia: residue analysis of chlorinated hydrocarbons in human bone marrow biopsy specimens by high resolution gas chromatography

In a first small study the chlorinated hydrocarbons present in the bone marrow of haematologically normal persons and of 6 randomly selected patients with severe aplastic anaemia have been identified and quantified by glass-capillary gas chromatography. We found great interindividual differences in the concentrations of these compounds both within the control group and within the patient group, but no definite difference between the two groups. So far, we have been unable to detect known toxic concentrations of chlorinated hydrocarbons in any of our samples, which could have supported the idea of a deficiency in the metabolism of these chemicals in some patients with aplastic anaemia.

[3,4-dichloroaniline--a nitrogen and carbon source for a mixed culture of microorganisms]

A mixed microbial culture assimilating 4-chloroaniline and 3,4-dichloroaniline as sole sources of carbon and nitrogen was isolated from soil treated with propanide for a long period of time. The process is accompanied with a 100% liberation of chloride ions. Aniline cannot serve as a growth substrate for the mixed culture. The authors have studied whether the mixed culture utilizing chloroanilines can decompose these compounds in model experiments with natural waters and soil suspensions. The culture actively decomposes 3,4-dichloroaniline (20 mg/litre) in natural water samples into which it has been added at a concentration of 10(3)--10(4) cells/ml. The decomposition is accelerated in experiments with suspensions of meadow-chernozem soils when cells of the mixed culture are added to the suspensions. The rate of 3,4-dichloroaniline degradation in a grey forest soil suspension remains nearly unchanged when the mixed culture is added to it. Degradation of the chloroaniline is accompanied with equivalent liberation of chloride ions when the mixed culture is added to natural water samples.

Effects of synthetic and natural toxicants on livestock

Synthetic and natural toxicants are constituents of soil, air, water and foodstuffs. Their impact on animal agriculture has resulted from acute and chronic intoxication and residues transferred into meat, dairy and poultry products. Recent advances in analytical chemistry and the sciences associated with toxicology have allowed better assessment of the hazard of toxicants on animals including man. Historically, natural toxicants (phytotoxins, mycotoxins and minerals) that are associated with many common feedstuffs accounted for toxicity episodes of epidemic proportions. Most synthetic chemicals (pesticides, nonpesticidal organic chemicals and drugs) have been introduced in increasing numbers since the 1940's. In the 1960's and '70's, recognition of the need to control their environmental distribution stimulated the introduction of numerous laws and regulations. In the last decade, several problematic synthetic chemicals have been banned, particularly those found to persist in the environment or those confirmed or suspected as carcinogens in humans. At the farm level, the development of various preventative management strategies has decreased the exposure of livestock to natural toxicants. In the future, the impact of natural toxicants on animal agriculture is expected to lessen as their existence, etiology and toxicology are determined. On the other hand, synthetic chemicals will continue to threaten animal health as greater numbers and quantities are released into the environment. These challenges should stimulate a greater involvement of animal scientists in toxicology.

[Key Rhodococcus enzymes in the catabolism of aromatic compounds]

The enzyme apparatus involved in the catabolism of aromatic compounds in rhodococci is characterized by the presence of pyrocatechase and protocatechoate-3,4-dioxygenase as principal enzymes cleaving the aromatic cycle. Metapyrocatechase was found in about 30% of the rhodococci. All the enzymes are inducible. The inductor of pyrocatechase seems to be cyc-cys-muconate, and that of protocatechase appears to be 3-oxoadipate. The metapyrocatechase of rhodococci, in contrast to that of Pseudomonas, is not induced by benzoate, p-toluylate, p-xylene and phenol. The activity of metapyrocatechase rises 20-50 times comparing to the basal level only in the presence of p-cresol. The enzyme has a relatively low activity in rhodococci (50-200 nmole per 1 min per 1 mg of protein), though a very high affinity for methylcatechols. The activity of metapyrocatechase with methylcatechols is 2-5 times as high as that with catechol as a substrate, whereas the activity of pyrocatechase with methylcatechols is two times as low as that with catechol as a substrate. Such additional substrates as acetate, glycerol or fumarate have no effect on the qualitative composition of the key enzymes involved in the degradation of aromatic compounds in Rhodococcus carollinus 172. Glucose represses the synthesis of enzymes cleaving the aromatic ring by 100%. Fumarate taken in a 5-fold excess inhibits the activity of catechol oxygenases by 40%; if it is taken in a 1000-fold excess, it inhibits the enzyme activity by 100%.

Investigations on trifluralin binding to soil and possible uptake of bound residues by plants

3H-labelled trifluralin was synthesized by condensation of 3H-4-chloro-3,5-dinitro-alpha,alpha,alpha-triflurotoluene with di-n-propyl-amine. The labelled herbicide was applied to autoclaved and non-autoclaved heavy, light and sandy soils at 30 ppm. The treated samples were incubated for 2 months, then extracted and analyzed for metabolic products and bound (nonextractable) 3H-residues. The amounts of bound residues were higher in autoclaved than in nonautoclaved and in sandy than in either heavy or light soils. The mono- and didealkylated products were detected as well as unknown polar derivatives. 3H-residues bound to preextracted soils were absorbed by bean plants. The roots contained (0.20 - 0.72 ppm) more residues than did the shoots (0.07 - 0.15 ppm) and the uptake was higher in the case of autoclaved especially sandy soils.

Epidemiologic study of physiological effects in usual and volunteer citrus workers from organophosphate pesticide residues at reentry

Biological parameters associated with organophosphate (OP) pesticide exposure were evaluated in a study of citrus harvesters. Changes in these parameters related to environmental residues of cholinesterase (ChE) inhibiting pesticides were studied. Further, it was determined whether usual and volunteer workers differed in their biochemical parameters after exposure to pesticide residues during field operations. Urine metabolite findings for usual farm workers showed that some exposure to OP pesticides had occurred. Too few workers were available during the reentry stage of the field study to evaluate the effect of this exposure on blood ChE. Baseline (nonexposure) acetylcholinesterase (AChE) and plasma cholinesterase (PChE) activities were significantly higher in usual (Mexican American) workers than in volunteer workers (student volunteers). Student volunteers in the test citrus grove showed statistically significant declines in PChE during the exposure period, yet changes in AChE activity, urine metabolites, and residue levels were very small. The implications of these findings are discussed in light of current reentry standards as well as future epidemiologic studies on reentry research.

Comparison of the content of chlorinated pesticide residues in human semen, testicles and fat tissues

Studies were undertaken to determine the residues of chlorinated pesticides in human semen, testicles and fat tissues. Samples of semen were taken from a random population and samples of tissues at surgery. A preliminary estimation was made to assess the level gradient of environmental pollutants in these types of specimens characterized by a different fat content. Pesticides were considered to show an affinity to fat. The results are an early stage in a programme for a detailed analysis of the effect of pesticides on human reproductive ability and on the value of genetic material.

Bioavailability of residues: current status

1. Bioavailability studies and current methodologies provide a very useful system for assessing the safety of radioactive drug related residues in food-producing animals. 2. Two year chronic toxicity studies provide useful information regarding the toxic properties of the parent drug but they have limited value for the assessment of safety for "second-pass" exposure of radioactive drug related residues of food-producing animals. The relevancy of the traditional two year toxicity study is seriously questioned as a method to determine the safety of drug related residues on the basis of a "second-pass" exposure. 3. More relevant toxicologic information can be gained by using the food-producing animal target species in toxicology studies. Comparative pharmacologic and toxicologic differences between laboratory animals and food-producing animals would be reduced.

Incorporation of foreign compound metabolites into plant and soil constituents

Recent developments in the technology of removal and characterization of bound residues of two herbicides, nitrofen and profluralin, from plants and soil will be reviewed. 14C-Nitrofen was found to be metabolized into starch in wheat grain as well as bound into the lignin fraction of wheat straw. Profluralin and its metabolites were found to be bound to field aged soil in several ways probably including hydrogen bonding and covalent bonding. The bound metabolites of profluralin in soil could be categorized according to the type of bonding occurring as well as to the physicochemical characteristics of the bound pesticide. By assessing the type of bonding involved between pesticide and plants or soil, some assessment of the bioavailability of these bound residues to the environment can be made. When applied to tissues of food producing animals, these techniques would help release bound drug residues and provide technology for characterization of the released material.

Classification of radioactive pesticide residues in food-producing animals

Modern analytical methodology, especially radiotracer techniques, makes it possible to detect residues of a foreign compound in animal products at levels seemingly impossible just two decades ago. Unfortunately, the ability to detect "total radioactivity" in a substrate does not assure that the chemical nature of the residues will be elucidated or that their toxicological significance can be properly assessed. Detection without identification has created a number of problems for the residue chemist and toxicologist. Among these is the problem of classifying radioactive residues in a manner that is meaningful to other scientists and which inherently denotes certain characteristics of the residues involved. While such a system is as yet incomplete, a general classification scheme has evolved in recent years that provides a common ground for categorizing radioactive residues, be they known or unknown. The system consists basically of four categories of radioactive residues: (a) free metabolites (b) conjugate metabolites (c) bound pesticide residues and (d) natural constituents. For the most part, the terms are self-explanatory, but precise definitions remain a point of debate among scientists. This paper discusses th criteria for classifying radioactive residues as perceived by the author based upon works of residue and metabolism chemists, especially those dealing with pesticide residues. Utimately, classification is dependent on the identification of total radioactive residues, a situation which is virtually impossible with many xenobiotics. Sound scientific judgment must remain the key ingredient in determining just how far one must go in identifying radioactive residues of drugs, pesticides and other chemicals which may become a component of the human diet.

Heptachlor residue accumulation and depletion in broiler chickens

Broiler chickens were fed heptachlor in concentrations of 0, 0.01, 0.03, 0.1, and 0.3 mg/kg of the ration for the first 8 weeks of life. There were no adverse effects on health or performance. Residue concentrations in adipose tissue increased rapidly for 1 to 2 weeks and then tended to plateau at concentrations about five times greater than those in the rations. Birds exposed only during the 7th week of life stored 25% as much residue as did the chickens exposed in the 1st week, but the former also ate only 63% as much feed. Fat extracted from liver and muscle contained more residue than did fat extracted from adipose tissue. The ratio of the concentration of heptachlor to its epoxide metabolite was higher in muscle fat than in heptic or abdominal fat. Residue concentrations depleted by about half in the first 4 weeks after cessation of exposure.

Relationship between biotransformation and the toxicity and fate of xenobiotic chemicals in fish

Many of the biotransformation reactions which have been described for xenobiotic substances in mammals have been demonstrated in fish in both in vitro and in vivo experiments. Several of these biotransformation reactions have been shown to occur in fish at rates which are sufficient to have significant effects on the toxicity and residue dynamics of selected chemicals. Inhibition of these reactions can lead to increased toxicity and bioaccumulation factors for certain chemicals. Several classes of compounds, including some polychlorinated biphenyls, are metabolized slowly, and their disposition in fish may not be influenced to any great extent by biotransformation. Metabolites of compounds which are biotransformed rapidly may appear in certain fish tissues, and in many instances these are not accounted for by conventional residue analysis methods. Microsomal mixed-function oxidases in several species of fish have been demonstrated to be induced by specific polycyclic aromatic hydrocarbons and by exposure of fish to crude oil. Induction of these enzymes in fish can result in both qualitative and quantitative differences in the metabolic disposition of xenobiotics to which fish are exposed.