The abilities of various doses of 3,3',4,4',5-pentachlorobiphenyl (PCB126) to induce changes in antioxidant enzyme activities and glutathione levels in the brain tissues of rats were examined in rats after subchronic exposure. Groups of rats were administered 10,30, 100, 300, 550 or 1000 ng PCB 126/kg/day, p.o., for 13 weeks and the activities of supeoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px), as well as (GSH) levels were determined in the brain tissue homogenates. Treatment resulted in significant and dose-dependent increases in the activities of the three tested enzymes. While maximal increase GSH-Px activity was achieved with a dose of 100-175 mg/kg/day, CAT and SOD activities continued to increase in response to maximal dose used for this study. GSH levels on the other hand, were suppressed significantly in a dose-dependent fashion. Data suggest that previously observed increase in oxidative stress production by PCB-126 in the brain tissues of rats is associated with dose-dependent rise in antioxidant enzyme activities and GSH depletion. However, the increases in the antioxidant enzyme activities can not provide full protection against oxidative damage induced by the same doses. In addition, GSH depletion plays a critical role in the previously observed oxidative stress in response to this compound.