The effect of thyroidectomy and hypophysectomy, and subsequent growth hormone therapy, on skeletal muscle growth was studied in rats by determing the wt. gain and chemical composition of the thigh muscle. The chemical analyses consisted of quantitative detn. of water, total N, fat, and the following N fractions: myosin, water-soluble protein, alkali-soluble stroma, collagen, and nonprotein N. Thyroidectomy at birth markedly impaired muscle growth. In addition to the subnormal muscle mass, the dwarfism was associated with a deficiency in the amt. of the myosin and water-soluble protein fractions. Growth hormone admn. increased the body-wt. gain 2-fold and the collagen percentage almost 2-fold, but it had no effect on the subnormal muscle/body-wt. ratio, and only a slight effect on the subnormal myosin content. Hypophysectomy at weaning retarded the growth rate of the thigh muscle, but did not alter the chemical composition of the muscle when compared to size-controls. Growth hormone admn. (total amt. equal to 1/8 of that given to thyroidectomized rats) provoked a marked wt. increase in the thigh muscle, accompanied by a significant increase in the myosin fraction. Aging in normal male rats, 23-107 days of age, was associated with an increase in the thigh-muscle/body-wt. ratio, and a decrease in the water/protein ratio. The increased N content of the muscle, relative to body wt., was due to marked increments in the myosin, water-soluble protein, and non-protein N fractions.