It is known that, upon combination with substrates or substrate competitors, succinate dehydrogenase is converted from an inactive to active form and that on removal of the activator the enzyme reverts to an inactive form. The activation is characterized by a high energy of activation (36 kcal/mole). It has been found that during the oxidation of NADH by inner membrane preparations a similar activation of succinate dehydrogenase occurs. The maximal extent of activation reached, the energy of activation of the process, and the kinetic properties of the activated form of the enzyme are the same as when activation is induced by the substrates or substrate analogs. When NADH is exhausted succinate dehydrogenase is rapidly deactivated. The activation-deactivation processes are sufficiently rapid at 37° to be of significance in metabolic regulation. Extraction of endogenous coenzyme Q (CoQ) from the membrane results in loss of activation by NADH but on reconstitution of the particles with respect to CoQ, activation by NADH is restored. These observations and studies with inhibitors indicate that NADH itself is not the activating agent but merely serves to reduce endogenous CoQ and that CoQH2 is the activating agent. It is suggested that CoQH2 is a positive modifier of succinate dehydrogenase. The site at which CoQH2 acts may not be the same as the one involved in electron transport from succinate dehydrogenase to CoQ since thenoyltrifluoracetone abolishes electron flux from the dehydrogenase to CoQ without affecting activation by CoQH2. The role of the activation-deactivation processes in electron flux between the dehydrogenase and the respiratory chain and in the control of theKrebs cycle are discussed.