The effects of two distinctive BHA pretreatment regimens on the biliary excretion of aflatoxin B1 (AFB) metabolites and the covalent binding of AFB to hepatic DNA were studied in vivo in the rat. To differentiate between enzyme induction effects and direct antioxidant effects, BHA was given to rats for 9 days (500 mg/kg/day, sc) or as a single dose (500 mg/kg, po), and [3H]AFB was administered ip. Repeated treatment with BHA enhanced the biliary excretion of both the glutathione conjugate of AFB and the AFP1-glucuronide to 200% of control values, reduced the amount of AFB remaining in the liver to 53% of control and reduced the covalent binding of AFB to hepatic DNA to 16% of control. A single BHA treatment had no effect on the biliary excretion of AFB or the binding of AFB to hepatic macromolecules, even though high concentrations of BHA were present in the liver during the period of AFB metabolism. These results support the hypothesis that BHA inhibits AFB carcinogenesis via the induction of phase II biotransformation pathways such as glutathione S-transferase, which act to reduce the amount of AFB-epoxide available for binding to DNA. We found no evidence of a direct antioxidant effect of BHA in altering the hepatobiliary disposition of AFB.