The purified succinate-ubiquinone reductase catalyzes the L- (or D-) malate: acceptor oxidoreductase reaction with Km for malate of about 2.10(-3) M and initial Vmax of 50 and 100 nmol per min per mg of protein for L- and D-stereoisomers, respectively (25 degrees C, pH 7.0). The reaction rate rapidly decreases both in the absence and presence of L-glutamate and L-glutamate-oxaloacetate transaminase added for trapping of oxaloacetate. Both keto and enol forms of oxaloacetate were found to be strong, slowly dissociating inhibitors of succinate dehydrogenase; the first-order rate constant for the enzyme inhibition by the enol form is about 3 times as high as that by the keto form. Oxidation of malate by succinate dehydrogenase in the presence of the oxaloacetate trapping system occurs at an indefinitely constant rate when enoloxaloacetate, which is an immediate product of the reaction, is rapidly converted into the keto isomer--a substrate for transaminase. A quantitative kinetic scheme for malate oxidation by succinate dehydrogenase which includes two kinetically distinct enzyme-oxaloacetate complexes is proposed, and the specific role of the mitochondrial oxaloacetate keto-enol-tautomerase (EC 5.3.2.2) in the regulation of succinate dehydrogenase is suggested.