Serum FSH, LH and prolactin were measured in fifty-eight normal males between 30 and 80 years of age. At the same time similar estimations were performed on samples taken from 232 patients with benign prostatic hypertrophy (BPH) and twenty-six patients with prostatic carcinoma. The three groups were compared with respect to age, and it was observed that significant rises related to age occurred in the serum levels of FSH, LH and prolactin in normal men after the sixth decade. The patient groups did not differ significantly from each other, or from the normal age-matched population in respect to prolactin and FSH levels. Serum LH in both the carcinoma and BPH patient groups, however, differed significantly from the controls, and remained at the level associated with younger normal males. It is suggested that testosterone metabolites from the prostate exert a negative feedback on pituitary LH secretion.