The effects of β-naphthoflavone (β-NF) and a chlorophenoxyacetic acid herbicide (MCPA) on hepatic and renal monooxygenase activities and conjugating enzymes from immature carp (Cyprinus carpio) were studied. β-NF increased hepatic monooxygenase activities but the patterns of differential induction generally obtained in rat liver microsomes with two series of homologous substrates, alkoxycoumarins and alkylresorufins, were not found to be similar in carp liver microsomes. On the other hand, MCPA caused no changes in oxidative metabolism, with the exception of decreased aryl hydrocarbon hydroxylase activity. Renal activities were not modified by MCPA, while β-NF treatment resulted in marked increases in monooxygenase activities with alkylresorufins as substrates. No changes were found in conjugation activities after treatment with MCPA or β-NF. These results indicate that (a) the herbicide MCPA should have no effect on drug-metabolizing enzymes from carp, and (b) the hepatic and renal monooxygenase activities of carp are responsive to β-NF, allowing their use in monitoring water pollution.