The short-term (6-day) endocrine effects of tamoxifen and testolactone were investigated in men with prostatic carcinoma. Tamoxifen treatment (20 mg/day) did not affect the gonadotropin levels, but it temporarily increased prolactin, induced sex hormone-binding globulin production, and suppressed peripheral serum progesterone, 17-hydroxyprogesterone, androstenedione, testosterone, and 5 alpha-dihydrotestosterone concentrations. These changes were attributed to the estrogenic properties of tamoxifen, since no changes in peripheral serum estradiol concentrations were observed. Testolactone (1000 mg/day) decreased peripheral estradiol concentrations by 50% and increased the concentrations of the neutral steroids measured. The increases in serum FSH and LH were very small. This study corroborates the early estrogen-like action of tamoxifen, and the experiment with testolactone further suggests that endogenous estradiol has physiological functions in man, regulating gonadotropin and androgen production.