1. 1. The rate and extent of exchange of external [14C]malate with endogenous counter ions in isolated rat heart mitochondria was found to depend markedly on the concentration and age of the isolated mitochondria. 2. 2. Ascorbic acid plus tetramethylphenylenediamine, or ATP stimulated malate exchange at low concentrations of mitochondria (0.2-0.8 mg protein per ml) but caused inhibition at higher concentrations. 3. 3. The rate of malate exchange was partially inhibited by atractyloside and benzene- 1,2,3-tricarboxylate, completely inhibited by 2-phenylsuccinate, 2-benzylmalonate and high concentrations of mersalyl, and stimulated by N-ethylmaleimide.