SUMMARY: The disposition of carbohydrate in isolated diaphragm from normal and hypophysectomized rats has been studied, after addition of acetate, acetoacetate or β-hydroxybutyrate ± ox growth hormone (GH) to the incubation medium in which glucose was always present. 1. In normal diaphragm acetoacetate alone depressed glucose uptake by about 20%, without inhibiting glycogen synthesis. There was a considerable inhibition of pyruvate formation. This effect was not seen in diaphragm from hypophysectomized rats. 2. Acetate alone inhibited glycogen synthesis in diaphragm from normal, but not hypophysectomized, rats. 3. β-Hydroxybutyrate alone inhibited pyruvate formation in diaphragm from hypophysectomized, but not normal, rats. 4. With normal diaphragm the addition of GH to the medium (1 μg/ml.) produced only slight changes in carbohydrate metabolism, although there was a significant stimulation of glucose uptake when acetate was present. With diaphragm from hypophysectomized animals GH at this concentration caused a striking increase in glucose uptake, accompanied by increased glycogen synthesis (insulin-like effect). In the absence of substrate other than glucose GH increased lactate output; in the presence of acetate, GH inhibited lactate formation. 5. Hypophysectomy did not affect the rate of uptake of acetoacetate, but the uptake of acetate was markedly increased. 6. In confirmation of previous work GH did not affect the uptake of acetoacetate by diaphragm from either normal or hypophysectomized rats.