Polychlorinated biphenyls (PCBs) have been widely utilized industrially, resulting in persistent human exposure. To investigate the long-term biochemical effects of bioretained PCB congeners, mice were injected once with Aroclor 1254 (100 or 500 mg/kg) and sacrificed at intervals from 48 hrs to 30 weeks. Hepatic cytochromes P450 1A1 and 2B1, assessed enzymatically and by immunoblotting, were significantly increased over 30 weeks after both doses. Carcass burdens of PCBs reached 15 and 80 ppm, respectively, with estimated half-lives of 25–30 weeks. Human exposure has yielded body burdens comparable to the low dose levels in this study, with an estimated half life of 30 months. These data suggest that intermittent exposures to PCBs elicit P450 ‘induction in liver of rodents at body burdens similar to humans’, and that exposure to higher levels would result in significant persistent biochemical effects.