The effects of the environmental pollutant benzo(a)pyrene (BP) were assessed on the hepatic mixed function oxygenase system (MFO) in the channel catfish,Ictalurus punctatus. Initially, the optimum conditions were established for measuring hepatic MFO in this species of fish. Parameters investigated were divalent metal ion requirement,in vitro incubation temperature for optimum activity, pH for optimum activity and the time course for enzyme induction. A single dose of BP (100Μg/kg body weight) given intraperitoneally to catfish kept in the laboratory at 19†C induced the hepatic MFO benzo(a)pyrene hydroxylase (BaPH) with maximum activity occurring 96 hr after the injection of BP. Cytochrome P450 assays were carried out at 24 hr and 48 hr after injection of BP. Hepatic cytochrome P450 and BaPH activity increased from 24 to 48 hr after the BP injection. In a temperature acclimation experiment, fish were maintained at 15†, 19†, and 26†C; assays for BaPH were conducted 24, 48, and 96 hr after an inoculation with BP. Induction occurred equally in all BP-injected fish by 24 hr; by 48 hr, induction was greatest in the fish kept at 26†C, least in the 15†C group. The fish kept at 19†C were intermediate. However, by 96 hr, induction had occurred to the same degree in all three experimental groups. BaPH activity was always greater in the BP-injected groups than in their respective control groups. Phenobarbital (75 mg/kg body weight) failed to cause an increase in hepatic dealkylating activities or cytochrome P450 concentration.