7-Ethylresorufin O-deethylase (EROD) activity and the cytochrome P-450 content of liver and kidney microsomes were measured in male and female nase (Chondrostoma nasus) from the River Rhône (France) caught downstream and upstream of a PCB incineration plant. Concurrently, PCB concentrations in the flesh of nase were also measured. The results showed that sex affected hepatic, but not renal, EROD activity during gonadal maturation. The male nase demonstrated higher activity than the female. Upstream/downstream comparisons clearly revealed a more elevated hepatic EROD activity in fish contaminated by PCB pollution than in fish captured in the reference area throughout the reproductive cycle, demonstrating the reliability of this enzymatic activity as a biochemical indicator. Renal EROD activity did not seem to be sensitive to PCB pollution, since neither male nor female nase showed significant upstream/downstream differences.