1. The mitochondrial malate dehydrogenase from rat liver has been purified to a state of homogeneity as judged by starch-gel electrophoresis and the cytoplasmic isoenzyme has been obtained in a partically purified state. 2. Inhibition of the isoenzymes by sulphite has been studied. 3. In mitochondria loaded with sulphite, the catalytic activity of the (partially inhibited) internal malate dehydrogenase has been measured by addition of oxaloacetate to the suspension medium and observation of the consequent decrease in fluorescence of NADH. 4. Addition of mitochondrial malate dehydrogenase to suspensions of mitochondria loaded with sulphite resulted in an increase in the level of intramitochondrial enzymic activity as measured by the above technique. Addition of the cytoplasmic isoenzyme did not result in such an increase. 5. These results show that mitochondria in suspension are permeable to the mitochondrial malate dehydrogenase but not to the cytoplasmic isoenzyme. 6. This conclusion has been confirmed by direct measurement of a decrease of enzyme activity in solution and an increase inside the mitochondria after incubation of organelles in solutions containing mitochondrial malate dehydrogenase. No such effect was observed with the cytoplasmic isoenzyme. 7. Some features of the permeation process have been studied.