The NADH-dependent activity by hepatic microsomes of Japanese monkeys for 7-oxo-Delta(8)-tetrahydrocannabinol (7-oxo-Delta(8)-THC) formation from 7beta-hydroxy-Delta(8)-THC exhibited about 70% of the NADPH-dependent activity (100%) at the substrate concentration of 72.7 microM, although NADPH was an obligatory cofactor for maximal activity. Both NADH- and NADPH-dependent activities were significantly inhibited by the typical P450 inhibitors, such as SKF525-A and metyrapone. Both activities were almost completely inhibited by the NADPH-P450 reductase inhibitor diphenyliodonium chloride. The ratio of NADH- and NADPH-dependent activities varied significantly according to the substrate concentration. Interestingly, the NADH-dependent activity was higher than that of NADPH at low substrate concentrations of 13-50 microM. The ratio was also affected by the cofactor concentration. In the reconstituted system of CYP3A8 purified from hepatic microsomes of Japanese monkeys as a major enzyme responsible for the NADPH-dependent oxidation, NADH as well as NADPH could sustain the oxidation of 7beta-hydroxy-Delta(8)-THC to the corresponding ketone. The NADH-dependent oxidation of 7beta-hydroxy-Delta(8)-THC by monkey livers is mainly catalyzed by CYP3A8 as well as the NADPH-dependent oxidation. These results indicate that NADH as a cofactor may be also useful for the oxidation of 7beta-hydroxy-Delta(8)-THC, and that the cofactor requirement for the reaction is varied by the concentrations of substrate and/or cofactor.