Among biotransformation reactions the oxidation reactions and induction and inhibition of these reactions are the most studied. Many organic (aromatic) compounds can be oxidized in fish by the cytochrome P-450 dependent monooxygenases. Enzymes catalyzing these reactions can be induced under environmental conditions either by "natural" or by anthropogenie compollnds. Monooxygenase activities vary up to 6OO-fold within the fishes and average activities are about 3-36% of average mammalian activities. In general conjugating enzyme activities of fish are also lower than in mammals. Activities of these conjugating enzymes can vary enormously and are very dependent on the substrates which have been used to measure these activities. If compared to oxidation reactions, much less is known about the two other types of phase I reactions: reduction and hydrolytic reactions. Of the phase 11 reactions the most extensively studied reactions are glucuronide, glutathione and sulphate conjugation. Biotransformation reactions are most abundantly studied in the liver. In generalIiver has highest activities per gram tissue and xenobiotics will be predominantly biotransformed there, since xenobiotics pass this organ after entering the organism. Other organs, however have high metabolie activities based on these whole organs, and thus can contribute significantly to metabolism of xenobiotic compounds. Metabolism can be infiuenced by several factors, such as route of administration, diet, temperature and/or season, hormones, age, sex, species and strain. The importance of these factors on biotransformation however, is still uncIear. Compounds which are metabolized relatively fast are those which can undergo conjugation, e.g. compounds with functional groups such as -COOH, -OH and/or -NH2. The most extensively studied compounds in metabolism studies in fish are aldrin, benzo(a)pyrene, carbaryl, chlorophenols, DDT and structurally related compounds, naphthalene, parathion, phenols and some PCBs. It may be cIear from the data presented here, that fish are able to metabolize a number of xenobiotic compounds and that biotransformation can infiuence bioaccumulation or detoxifying or activating processes significantly. Hitherto, however only a limited number of degradation pathways have been elucidated, while very little is known about the bioaccumulation and toxicology ofthe metabolites which are formed. Hence a lot of research is required to increase the understanding of the role which metabolism plays in the bioaccumulation and toxicity of organic compounds and their biotransformation products. © Springer-Verlag Berlin Heidelberg 1989.