The two components of the protonmotive force, the pH gradient (delta pH) and the transmembrane electric potential (delta psi), were measured in rat liver mitochondria as a function of K+ concentration in the suspending medium. It was found that both the rate of formation and the final level of delta pH upon energization of mitochondria with succinate increased with increasing [K+]. Concomitantly, delta psi decreased so that the level of the protonmotive force remained practically unchanged. Potassium channel opener RP66471 further potentiated both the formation rate and the level of delta pH. These results are interpreted as showing that the electrophoretic K+ influx enables the formation of delta pH by partly compensating charge transfer due to the proton pumping.