Univalent oxidation-reduction reactions coupled with the menadione (MK)/menadione semiquinone (MK.-) system were investigated by using microsomal flavin enzymes. NADPH-cytochrome P-450 reductase gave a dynamic equilibrium of oxidation-reduction of cytochrome b5 in the presence of menadione (MK), the level of which depended on the concentration of O2 and superoxide dismutase. The data suggest that the superoxide and menadione radicals are involved as an active intermediate in this system. The overall reaction at steady state appears to be composed of four main reactions, eqs. 2-5, and eqs. 2 and 4 are in equilibrium.