Juvenile rainbow trout were exposed to unlabelled benzo[a]pyrene BaP and (3)H benzo a pyrene ((3)H BaP), in a static exposure system for 2 days. The initial concentration was 30 μg l(-1) and 0.625 μCi l(-1), corresponding to 6 mg kg(-1) body weight and 125 μCi kg(-1) body weight. Hepatic 7-ethoxyresorufin-O-deethylase (EROD) activity was measured during the exposure and depuration periods, elucidating the time course pattern of CYP1A induction. Maximum induction (11-fold) of EROD activity was observed on day 2 after addition of BaP to the water. Tissue distribution of (3)H-BaP was studied by liquid scintillation counting and whole body autoradiography. The concentration of (3)H-BaP-derived radioactivity was highest in the bile at all sampling times. High levels of radiolabelled compound were also present in the gills, liver and the olfactory organ. There was an overall decrease in all tissues during the depuration period. The elimination of (3)H-BaP-derived radioactivity from the gills, however, was slow compared with liver and blood (6.2 days vs 2.7 and 2.9 days, respectively).