Brown bullheads (Ameiurus nebulosus) were collected from the St. Lawrence River and compared in their detoxication capacities to bullheads from a relatively non-polluted aquatic system, Lac La Pêche. The content of polychlorinated biphenyls (PCBs) in white muscle was significantly higher (22-fold) in bullheads from the St. Lawrence River compared with those from Lac La Pêche. Activities of liver ethoxyresorufin O-deethylase (EROD), a common Phase I enzyme, were 2.8-fold higher in St. Lawrence River bullheads than in fish from La Pêche. Conjugation activity by hepatic glutathione S-transferase (GST) was 3-fold higher in the St. Lawrence River fish, and significantly higher activities were also detected in kidney and white muscle in these fish as compared with the Lac La Pêche group. UDP-Glucuronosyltransferase (UDPGT) activities in liver and kidney did not differ between the two groups. Activities of cytosolic superoxide dismutase (SOD) were significantly higher, while lower activities of catalase (CAT) in kidney and glutathione peroxidase (GPX) in red and white muscles were noted in the St. Lawrence River bullheads. Concentrations of total glutathione (TGSH) in the different tissues revealed significantly lower levels in liver, kidney and white muscle of bullheads from the St. Lawrence River. The changes in Phases I and II enzymes and TGSH levels in the various tissues relate to higher PCB concentrations of muscle tissue and suggest activation of detoxication capacities, but weakened antioxidant status in the bullheads from the polluted area. The results implicate the involvement of white muscle along with liver and kidney in the whole fish detoxication process.