Oxidative metabolism of carbamazepine results in covalent binding of its reactive metabolite to liver microsomal proteins, which has been proposed as an important event in pathogenesis of the hypersensitivity reactions to this drug. Although the proposed reactive metabolites are produced by cytochrome P450 enzymes (P450 or CYP), the impact of the formation of unstable metabolites on the enzyme itself has not been elucidated. The present study examines the alteration of P450 enzyme activities during the metabolism of carbamazepine. Liver microsomes from rats and humans were preincubated with carbamazepine in the presence of NADPH, and subsequently assayed for monooxygenase activities representing several P450s. No evidence was obtained for inactivation of CYP2C11, CYP3A, CYP1A1/2 or CYP2B1/2 in rat liver microsomes during the carbamazepine metabolism, whereas the CYP2D enzyme was inactivated in a manner related to the preincubation time. Interestingly, under the same protocol human liver microsomes did not exhibit inactivation of CYP2D6, as well as there being no CYP2C8, CYP2C9 or CYP3A4 inactivation, whereas CYP1A2 was inactivated. Reduced glutathione could not protect against the observed inactivation of the P450s. These results suggest that CYP2D enzyme(s) in rats and CYP1A2 in humans biotransform carbamazepine into reactive metabolites, resulting in inactivation of the enzyme themselves, and raise the possibility that the P450 isoforms participate in toxicity induced by the drug in both animal species.