THE ENERGY requirements of cells are largely provided by carbohydrate metabolism. Because differences in energy requirements of tissues within the membranous labyrinth may provide information about physiological function, a considerable amount of work has been devoted to localization of enzymes involved in the metabolism of glucose. Most of the energy derived from the metabolism of glucose is liberated during the Krebs cycle. The Krebs cycle is the second part of a two-stage process of glucose metabolism. During the first stage glycogen is catabolized to pyruvate by phosphorylation. The pyruvate then enters the Krebs cycle where it is oxidized and ultimately converted to carbon dioxide and water. This dehydrogenation process takes place in several steps, each step being controlled by a specific enzyme. One of the essential steps in this sequence of reactions is the dehydrogenation of succinate by the enzyme succinic dehydrogenase (SDH). The hydrogen in this step is transferred