ExtractNineteen male subjects were given five daily injections of 17β-estradiol and circulating levels of estradiol (E2), testosterone (T), and gonadotropins were determined by radioimmunoassay before, during, and after the steroid course. Peak levels of E2attained during the 5 days of treatment ranged from 173–577 pg/ml. Four of seven normal adult men and one castrate man demonstrated suppression of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) with a subsequent rise in LH (positive feedback) while E2levels remained elevated. A rise in T was associated with the LH increment in the four normal men. Nine pre-, early, or midpubertal boys and two men with hypogonadotropic hypogonadism displayed only gonadotropin suppression after E2administration. The difference in LH response to estrogen (i.e., positive feedback) between the adult men with normal or elevated gonadotropin levels as compared with the endocrinologically normal boys is significant (P< 0.01).SpeculationPositive feedback between estrogen and LH is present in intact adult men, although the magnitude and consistency of this response is less than in women. Testosterone in men may act like progester-one in the female rhesus monkey to blunt the magnitude of the LH rise during estrogen administration. The potential for positive feedback appears to be a maturational event occurring during puberty in men as well as in women.