The protective effect of dicarboxylates on the active-site-directed inhibition of the membrane-bound succinate dehydrogenase by N-ethylmaleimide, steady-state kinetics methods for Ki and Ks determinations, and equilibrium studies were employed to quantitate the relative affinities of succinate, fumarate, malonate and oxaloacetate to the reduced and oxidized species of the enzyme. A more than 10-fold difference in the relative affinities of the reduced and oxidized succinate dehydrogenase to succinate, fumarate and oxaloacetate is found, whereas the reactivity of the active-site sulphydryl group does not depend on the redox state of the enzyme. The redox-state-dependent changes in the affinity of the membrane-bound succinate dehydrogenase to oxaloacetate can be quantitatively accounted for by a 10-fold increase in the rate of dissociation of the enzyme-inhibitor complex which occurs upon reduction of the enzyme. The data obtained give no support for either the existence of a sulphydryl group other than the active-site one important for the catalysis or for the presence of a separate dicarboxylate-specific regulatory site in the succinate dehydrogenase molecule.