Abstract: Anabolic activity of steroids may be assayed by body weight gain, nitrogen reten- tion, renotropic activity in castrated mice, or by increase in the weight of the levator ani muscle of castrated rats. However, the levator ani-bulbocavernosus muscles of castrated mice have not been employed for this assay. In view of the above situation the effects of anabolic steroids upon the weights of the kidneys, levator ani and bulbocavernosus muscles of the castrated mice were investi- gated. The relative response in weights of the levator ani and seminal vesicles of cas- trated mice and rats and the levator ani-bulbocavernosus muscles of castrated mice to the administration of 17-α-methyltestosterone (MT) was also studied. Results indicate that the levator ani and bulbocavernosus muscles of the castrated mice were highly sensitive to anabolic steroids. The dose required to produce a response in the levator ani-bulbo- cavernosus muscles was smaller and the response appeared earlier and lasted longer as compared with those of the kidneys. The sensitivity of the levator ani-bulbocavernosus muscles and seminal vesicle test in castrated mice are similar to that of the levator ani and seminal vesicle test in castrated rats, as the response elicited in mice and in rats are about equal with approximately equal doses. The weights of the levator ani-bulbocaver- nosus muscles or seminal vesicles in castrated mice increased with increasing doses of MT. The ratios of the weight of levator ani-bulbocavernosus muscles and seminal vesicle of castrated mice receiving different dosages of MT were also relatively constant. Hence, it may be concluded that for the assay of anabolic activity the combined weight of the levator ani and bulbocavernosus muscles of castrated mice is a better criterion than the kidney, and the levator ani-bulbocavernosus muscles and seminal vesicle test in castrated mice may be used in place of the levator ani and seminal vesicle test in castrated rats.