Polychlorinated biphenyls (PCBs) have previously been shown to be inhibitors of carcinogenesis in trout. The mechanism of this inhibition was investigated by studying the effects of PCBs on aflatoxin B1 (AFB1) distribution, metabolism and DNA adduct formation, both in vivo and in vitro. A 24 h distribution study of injected tritiated AFB1 showed more radioactivity in blood, liver and bile in fish fed PCBs, but less in residual carcass. The metabolites of AFB1 found in vivo in blood plasma and liver homogenates were shifted by PCB pretreatment towards greater production of the polar metabolite aflatoxin M1 (AFM1) and glucuronide conjugates. The major metabolite in bile of PCB fish was the glucuronide of aflatoxicol M1 (AFL-M1), which was enhanced 15-fold over controls. Levels of aflatoxicol (AFL) glucuronide, the major conjugate in controls, were unaltered by PCBs. The pattern of AFB1 metabolism in isolated hepatocytes from PCB-prefed fish was consistent with in vivo metabolism. AFB1--DNA adduct formation in a 1 h assay was similar in hepatocytes from PCB-fed and control fish. However, the total rate of AFB1 metabolism was significantly elevated in hepatocytes from PCB-fed fish such that the degree of AFB1--DNA adduct formed per unit AFB1 metabolized was 42% lower than control. Similarly, adduct formation in vivo during the first 24 h post-AFB1 injection in PCB fish was not significantly different from controls. However, over a longer 21 day period, adduct levels in PCB fish were only 48-69% of controls (P less than 0.005, analysis of variance), once peak adduct formation was reached. Thus, initial rates of adduct formation may be misleading in the absence of further information on rates of carcinogen metabolism in vitro and/or pharmacokinetics of peak adduct formation in vivo. These results indicate that PCB inhibition of AFB1 carcinogenesis in trout involves dramatic initial changes in carcinogen distribution, metabolism and elimination which, over time, results in a net reduction of DNA adduct formation.