Half-lives of protein in cell fractions of liver, kidney and spleen from adrenalectomized and gonadectomized mice were determined. The effect of fasting, protein-free and high-protein diet and cortisol administration to adrenalectomized animals was also examined. Decrease in turnover rate of protein was observed in all tissues examined from adrenalectomized and gonadectomized mice, however the degree of decrease was more remarkable in tissues of adrenalectomized mice than in those of gonadectomized mice. Daily administration of a large dose of cortisol accelerated the degradation of labeled proteins. Fasting caused a remarkable retention of protein in liver and kidney but not in spleen and no significant change in turnover rate of proteins was observed in the sham-operated control group, therefore, it was concluded that the changes in half-lives observed in adrenalectomized and gonadectomized mice were mainly attributable to the endocrine deprival and not to the result of change in food intake after the operation.