Evidence from various sources demonstrates that the release of ATP in exchange for the entry of ADP across the mitochondrial membrane is an active process requiring energy. The necessary energy may be derived from the same source of energy as that used for oxidative phosphorylation. The following results will be discussed:--(1) The exchange is asymmetric with respect to the specificity of ADP and ATP in 'energized' mitochondria. From the outside ADP is much preferred to ATP, but from the inside both exchange with equal specificity. This asymmetry is abolished by de-energization of the membrane. (2) The ADP-ATP exchange is about 50% electrogenic: about half the ATP released against ADP is protonated. The excess of negatively charged ATP might prevent ADP from entering mitochondria against a membrane potential. (3) The ratio of ATP to ADP across the inner mitochondrial membrane is higher outside than inside only in the energized state. Variation of the ATP/ADP ratio maintains this difference. (4) The ADP/ATP ratio apparently varies with changes in the membrane potential as measured by Rb+-distribution. The correlation factor between deltaE changes and this ratio is 0.5, in agreement with predictions from proton-stoichiometry measurements. The deltapH does not significantly change the distribution ratio. (5) By following the P/O ratio, one can show that energy derived from expelling ATP against the ADP/ATP gradient lowers the amount of ATP synthesis. The P/O ratio is lowered as the imbalance of ATP to ADP increases. (6) The energy difference of the phosphorylation potential of ATP is calculated by various methods to be about 8-12 kJ, depending on the conditions. This free energy is the result of the 'active' transport which corresponds to the release of ATP outside the mitochondria. (7) In vivo studies on the distribution of ADP and ATP inside and outside the mitochondria in liver show a corresponding ratio difference of about 15 as predicted from the in vitro studies.