Fifteen patients with hypogonadism due to testicular, pituitary, or hypothalamic failure were studied. After a pretreatment period without substitution, patients received intramuscular injections of testosterone enanthate, equivalent to 25, 50, 100, and 250 mg testosterone, or placebo. Each dose was given for 4 weeks, with injections given every 2 weeks. All patients with plasma testosterone values below 2 ng/ml during the pretreatment period reported impaired sexual function. They responded to testosterone injections (50, 100, and 250 mg) with improvement of sexual behavior, as rated by sexual desire and frequency of erections and ejaculations. In the range between 2.0 and 4.5 ng testosterone per ml, four patients reported high frequencies of erections and ejaculations that did not change after testosterone treatment. Four other patients with testosterone values in the same range reported impaired sexual behavior and were successfully treated with testosterone enanthate. These data indicate that male sexual behavior is testosterone dependent and that the individual limit of plasma testosterone below which sexual behavior is impaired lies between 2.0 and 4.5 ng/ml.