IT HAS been known since the work of Dye and Maughan (1929) that the succinoxidase activity of skeletal muscle was diminished by thyroidectomy. Several groups of workers have shown liver succinoxidase to be sensitive to the absence or presence in excess of the thyroid hormone in the same direction as the energy metabolism (cf. Barker, 1951). In contrast, brain succinoxidase was reported by Rossiter (1940) to be unchanged by administration in vivo of desiccated thyroid, just as the oxygen consumption of this tissue was unaltered in experimental hyperthyroidism. Recent work has revealed that other tissues, like brain, exhibit no variation in energy metabolism under the influence of thyroxine or thyroidectomy (Barker and Schwartz, 1953). In order to determine how consistent the correlation between changes in oxygen consumption and succinoxidase activity would be, the present series of experiments was undertaken, employing tissues with and without oxygen consumption responses to thyroid status.