Mechanism-based inactivators of rat liver cytochrome P450 2B1 and 2B2 were used to evaluate the role of these enzymes in the hepatotoxicity of cocaine. Loss of liver microsomal androstenedione 16 beta-hydroxylation was monitored to determine the extent of P450 2B1/2 inactivation by chloramphenicol (CAP) or its 2B-selective analogue, N-(2-p-nitrophenethyl)chlorofluoroacetamide (pNO2C1FA). The effect of P450 2B1/2 inactivation on cocaine-mediated hepatotoxicity was assessed in rat liver slices. Exposure of slices from phenobarbital-induced Lewis rats to CAP concentrations ranging from 100 to 500 microM resulted in a concentration-dependent decrease in P450 2B activity and a corresponding decrease in cytotoxicity as measured by K+ loss following exposure to 1 mM cocaine. Treating slices from PB-induced rats with 250 microM pNO2C1FA protected slices against cocaine-mediated cytotoxicity after exposure to 500 microM cocaine. In vivo administration of 300 mg/kg CAP or 200 mg/kg pNO2C1FA to phenobarbital-induced Lewis rats decreased androstenedione 16 beta-hydroxylation to 30 or 39% of control, respectively, and blocked cocaine-mediated K+ loss in rat liver slices. Rat liver microsomes from animals treated with either CAP or pNO2C1FA displayed approximately 40% of the control rate of cocaine N-demethylation. Experiments with phenobarbital-treated Munich Wistar (WM) rats, which lack 2B2, revealed similar rates of microsomal N-demethylation and comparable in vitro hepatotoxicity to Lewis rats. The capacity of a specific P450 2B1/2 inactivator to protect against cocaine-mediated hepatotoxicity both in vivo and in vitro and the results with the WM rats support the identification of P450 2B1 as a major cocaine bioactivating form.