1. A spectrophotometric assay of the rates of penetration of oxaloacetate and l-malate into mitochondria is described. The assay is based on the measurement of the oxidation of intramitochondrial NADH by oxaloacetate and of the reduction of intramitochondrial NAD(+) by malate. 2. The rate of entry of both oxaloacetate and l-malate into mitochondria is restricted, as shown by the fact that disruption of the mitochondrial structure can increase the rate of interaction between the dicarboxylic acids and intramitochondrial NAD(+) and NADH by between 100- and 1000-fold. 3. The rates of entry of oxaloacetate and malate into liver, kidney and heart mitochondria increased by up to 50-fold on addition of a source of energy, either ascorbate plus NNN'N'-tetramethyl-p-phenylenediamine aerobically, or ATP anaerobically. 4. In the absence of a source of energy the changes in the concentrations of intramitochondrial NAD(+) and NADH brought about by the addition of l-malate or oxaloacetate were followed by parallel changes in the concentrations of NADP(+) and NADPH, indicating the presence in the mitochondria of an energy-independent transhydrogenase system. 5. The results are discussed in relation to the hypothesis that malate acts as a carrier of reducing equivalents between mitochondria and cytoplasm.