Measurements of radioactivity were made on various tissues of the male mouse and the rates of in vivo incorporation of various radioactive amino acids into proteins were determined in intact, castrated, and male hormone-treated castrated mice. Using the rate of incorporation of radioactive amino acids into protein tissue as a criterion for protein metabolism, male hormones at physiological dosages produced no significant protein anabolic effect in visceral and muscle tissues of the mice studied. As a general rule, the castrated mice incorporated more radioactive amino acids than intact animals. Incorporation of radioactive histidine however in longer time periods appeared to follow a somewhat different pattern, i.e., the male hormone showed a stimulating action in both intact and hormone-treated castrates.