A sensitive homologous radioimmunoassay for mouse GH capable of measuring concentrations of GH as low as 1 ng/ml of mouse serum has been developed. This assay has been used to study GH secretion in C3H mice under several physiological conditions. Newborn mice possessed very high concentration of GH in serum which persisted for two weeks. Adult male mice had more GH in their pituitary (66.9 μg/mg) and serum (12.0 ng/ml) than comparable females (48.9 μg/mg; 3.4 ng/ml). Ovariectomy in female mice increased pituitary and serum concentrations of GH to male levels whereas stilbestrol administration decreased it. Orchiectomy or stilbestrol treatment of male mice resulted in decreased GH levels. During the estrous cycle, maximum levels of GH were attained during proestrus. Insulininduced hypoglycemia or treatment with L—dopa failed to induce rise in serum concentrations of GH in mice.(Endocrinology91: 784, 1972)