O2 uptake in glucose-Ringer and anaerobic glycolysis of cerebral cortex and liver slices were measured in manometers, using rats unoperated or hypophysectomized, with and without treatment with thyrotrophic hormone (100-2400 u. over 1-16 days). After hypophysectomy, O2 uptake of brain slices was not appreciably diminished, compared with normals (contrary to the behavior of liver) while anaerobic glycolysis was increased about 25%. 5-8 days after starting treatment with thyrotrophic hormone, the O2 uptake of brain slices from hypophysectomized rats was increased, that of liver slices restored, as compared with normal rats. After 16 days'' treatment, the effect on brain was absent and that on liver less apparent, due, presumably, to the development of "antihormones." The treatment had little effect on the brains of unoperated animals.