Roach were sampled in the Seine River along a gradient of polyaromatic hydrocarbon (PAH) concentrations at three stations: Marnay upstream of Paris and Epinay and Poses downstream of Paris. Two hepatic monooxygenase activities: EROD (ethoxyresorufine-O-deethylase) and AE (aldrin epoxydase) and muscle residues of PCBs and PAHs were investigated during three periods of the year (before spawning, during spawning, and postspawning). Before spawning, EROD and AE activities were significantly correlated with muscle PCB levels (p </= 0.05 for both activities) and with muscle PAH levels (p </= 0.05 for both activities) at the three stations. Cd was not inversely related to monooxygenase activities. During spawning, enzymatic activities remained low and were not correlated to pollutant levels. Postspawning EROD and AE activities were again significantly related to muscle PCBs (p </= 0. 05 for both activities). Thus, investigations of EROD and AE activities may be considered as a reliable tool for biomonitoring of PCB or PAH exposure, as far as appropriate conditions are observed. http://link.springer-ny. com/link/service/journals/00244/bibs/37n2p242.html