NADPH-cytochrome-P450 reductase both purified from rat hepatic microsomes and involved in microsomal fraction was inactivated by treatment with alpha-lipoic acid. Since alpha-lipoic acid contains disulfide bond, it reacts with SH-groups of the reductase via the reaction of thiol-disulfide exchange resulting in the loss of the enzyme reducing activity. NADP+ completely protected reductase from the inactivation. The modification of reductase was reversible: the modified enzyme was partially reactivated with dithiothreitol and dihydrolipoic acid in the case when cytochrome c was used as a substrate of reductase. In the case when inorganic substrate, K3Fe(CN)6, was used for assay the activity of modified reductase no reactivation was observed. It was found that the order of the reaction of inactivation of membrane-bound microsomal reductase is equal to 1.2 +/- 0.2, which is in an agreement with pseudo-first order kinetics, and the second-order-rate constant of 26 M-1min-1. The results have shown that well known therapeutic agent alpha-lipoic acid is an efficient inhibitor of both purified and microsomal reductase.