Based on the premise that testosterone, but not 7 alpha-methyl-androgens, is reduced at the 5 alpha-position in the prostate and seminal vesicles, the differential bioactivities of these androgens were investigated in castrated rats. The ability of 7 alpha-methyl-19-nortestosterone acetate (MENT) to increase the weights of ventral prostate and seminal vesicles of castrated rats was four times higher than that of testosterone, while its effect on the weights of bulbocavernosus plus levator ani muscles (muscle), was 10 times that of testosterone. MENT was also approximately 12 times more potent than testosterone in the suppression of serum gonadotropin levels. A dose of testosterone that maintains serum gonadotropin levels and muscle mass also maintains prostate and seminal vesicle weights in castrated rats. By contrast, a dose of MENT that maintains muscle and gonadotropins does not maintain prostate and seminal vesicles. The action of other 7 alpha-methylated androgens were similar to that of MENT. The importance of 5 alpha reductase in the differential action of testosterone and MENT on prostate was confirmed by using a 5 alpha-reductase inhibitor. The activity of testosterone was significantly suppressed in the ventral prostate and seminal vesicles but not on muscle by the 5 alpha-reductase inhibitor (N,N-diethyl-3-oxo-4-aza-5 alpha-androst-1-ene-17 beta-carboxamide). The enzyme inhibitor, however, had no influence on the activity of MENT on either tissue. In contrast, cyproterone acetate, an antiandrogen that competitively binds to the androgen receptors, inhibited the action of MENT and of testosterone on the prostate as well as on the muscle. In conclusion, these observations show that 7 alpha-methylated androgens can maintain muscle mass and normal gonadotropin levels in androgen deficient rats without hyperstimulating the prostate. These findings suggest that 7 alpha-methylated androgens may offer some health benefits to men who require androgen treatment.