Previous work indicates that the administration of male hormone substance produces behavioral as well as organic changes. In baby chicks, aggressiveness, strutting and crowing have been induced. In man, greater capacity for penile erection and increased aggressiveness, energy and self-assurance have been noted. This paper reports additional data on mental and behavioral changes during treatment with testosterone propionate of 2 adult castrates, 2 cases of hypogonadism and 2 cases of apparently psychic impotence. Testosterone propionate was administered subcutaneously from 3 times weekly to once daily in doses of 20 mg dissolved in 1 cc of peanut oil. As controls, of which the patients were not informed, injections of oil without hormone were given for periods before and during treatment. Before treatment all patients were definitely lacking in any conscious goading sex drive, in the capacity for penile erection and even in the ability to enjoy sexual sensations from acts such as kissing and embracing. While the 4 patients who had once been normal could occasionally muster enough penile erection to effect vaginal entrance, these were too brief and weak to bring satisfaction to either their wives or themselves. With complete absence of either penile sensitivity or general emotional responsiveness, they reported that intercourse had become mere “arduous work.” All patients were disturbed, anxious and broken in spirit. They ranged from moderate to severe states of mental depression. The 4 definitely organic cases displayed additional symptoms. These patients suffered from vasomotor disturbances (hot flashes), such as women may experience at the menopause, and manifested very definite emotional instability characterized by sudden uncontrollable shifts in mood, by tendencies to break down into tears, by periods of extreme irritability and, sometimes, by sullen anger.