1. The effects of 3-methylcholanthrene, 2,3-benzanthracene and beta-naphthoflavone on xenobiotic metabolism in rainbow trout were studied. 2. These three polycyclic aromatic hydrocarbons increase hepatic arylhydrocarbon (benzo[alpha]pyrene) hydroxylase activity without altering glucuronyl transferase activity. 3. All three polycyclic aromatic hydrocarbons increased hepatic microsomal cytochrome P-450 levels by approximately 50%. 4. Pretreatment of trout with 2,3-benzanthracene resulted in an increase in the metabolism and biliary excretion of 2-methylnaphthalene in vivo. 5. These studies demonstrate that the induction of mono-oxygenation by polycyclic aromatic hydrocarbons can result in significant effects upon the metabolism and excretion of xenobiotics by fish in vivo.