Incubation of intact mitochondria with aspartate aminotransferase results in efflux of malate dehydrogenase and vice versa. The export process is specific and rapid. It shows saturation kinetics with respect to the effector enzyme consistent with involvement of a receptor for the effector in the mitochondrial membrane system. Export is inhibited by both beta-mercaptoethanol and by the metal chelating agent bathophenanthroline; both substances inhibit release of malate dehydrogenase by aspartate aminotransferase competitively whereas for release of aspartate aminotransferase by malate dehydrogenase inhibition is non-competitive. The efflux process is dependent on a trans-membrane pH gradient. Exported enzymes differ from the native forms in their dependence of activity on pH. Export of both aspartate aminotransferase and malate dehydrogenase is effected by incubation of mitochondria with the newly-synthesised precursor of aspartate aminotransferase; this observation provides supporting evidence for the physiological significance of the other results reported here. It is speculated that exported enzymes are on a pathway to degradation, and that coupled uptake and export is involved in the co-ordination of synthesis and breakdown of mitochondrial proteins.