Direct measurement of the electrogenic activity of purified mitochondrial transhydrogenase has been carried out. To this end, beef-heart transhydrogenase was isolated and reconstituted with phospholipids to form proteoliposomes. The transhydrogenase proteoliposomes were incorporated into a membrane filter impregnated with a decane solution of phospholipids. It is shown that addition of substrates of either the forward (NADPH and NAD+) or the reverse (NADH and NADP+) transhydrogenase reaction gives rise to an electric potential difference across the proteoliposome-treated membrane filter. The electric vector depends upon the direction of the reaction. The proteoliposome-supplemented compartment charges negatively in the case of the forward reaction and positively in the case of the reverse one. Addition of the reaction products after substrates equalizes the potentials. The transhydrogenase-treated membrane filter retains the ability to perform transhydrogenase-linked electrogenesis after removal of excess non-incorporated proteoliposomes. The electric potential difference reaching 20 mV immediately after the transhydrogenase substrate addition, slowly decreases due to accumulation of the reaction products. Such decay is prevented when the mixture is supplemented with the substrate-regenerating and product-utilizing enzymic systems. Under these conditions, a steady continuous electric current of about 10 pA can be observed.