FMN(H-2) has been found to work as a two-way mediator of diaphorase to reduce NAD(+) as well as to oxidize NADH. The FMN/diaphorase/NAD system has been successfully combined with lactate dehydrogenase-catalyzed reduction of pyruvate and oxidation of lactate. The thermodynamics of the two-way bioelectrocatalytic system is described.