The relation between the intramitochondrial and extramitochondrial ratio ATP/ADP, the transmembrane potential and pH gradient is investigated in the present communication. For this purpose mitochondria are equilibrated with added [14C]ATP in the presence of substrate and oligomycin for eliminating phosphate transfer by ATPase. The membrane potential was measured by the distribution of 86Rb+ in the presence of valinomycin, the deltapH by the distribution of [14C]acetate. In the energized state by varying deltapsi between 60 and 160 mV, the internal (ATP/ADP)i is decreased 30-fold, the external (ATP/ADP)e remains largely constant. As a result, the deltalog (ATP/ADP)e/(ATP/ADP)i = deltalogphi is increased linerly with deltapsi according to the following relation: deltalogphi = 0.85 deltapsi - 0.35. The deltapH was changed between 0.1 and 0.8 by increasing the Pi concentration causing only a minor decrease of deltalogphi would be expected if the ATP-ADP exchange has a significant electroneutral portion. Also in the uncoupled and respiration-inhibited state the same function between deltalogphi and deltapsi is found as in the energized states. It is concluded that under these conditions the ATP-ADP exchange is largely electrical.