The protein anabolic effect of testosterone and related steroids is expressed by the synthesis of protein in many extragenital tissues in addition to the accessory sex organs. The main site of extragenital actions is the skeletal muscles. The degree of response of the muscles, however, varies widely among the individual muscles and also among species. The response of the ‘levator ani’ (properly the dorsal bulbocavernosus) has been used as an assay method. Comparison of the response of this muscle with that of the seminal vesicles and/or the prostate of the immature castrated rat has suggested a large number of steroids with preferential ‘levator ani’ activity. The use of this muscle as representative of other skeletal muscles has been severely criticized. Furthermore, the apparent separation of the two activities in the animal experiments has not been supported by clinical trials. Consequently, nitrogen balance assays have been made in the castrated rat, in female ovariectomized monkey and in man. Many steroids were found to be more active than testosterone propionate or methyltestosterone. A separation of the action of a steroid on extragenital tissue from that of the accessory sex organs will probably not be achieved until the mechanism of action in the two types of tissue is elucidated.