Ethinyl estradiol (50 micrograms/day) or fluoxymesterone (10 or 20 mg/day), chosen because each is orally active and because fluoxymesterone is probably not converted to an estrogen, were given alone and in combination to adult men over several weeks. Measurements were made of serum FSH, LH, testosterone, and estradiol. The estrogen given alone suppressed serum FSH while the androgen given alone did not; however, the androgen may have enhanced the suppressive effect of the estrogen on the serum FSH. Neither steroid alone changed the serum LH but both together suppressed it. The estrogen alone decreased the serum testosterone, an effect probably mediated by the concomitant fall in serum FSH and a resulting decrease in sensitivity to the constant level of LH; a direct effect of estrogen on the testis seems less likely. The doses of estrogen and androgen used probably had a biologic effect equal to or somewhat above that of endogenously produced estrogen and androgen and thus reflected the maximum physiological effects of the endogenous steroids. Thus, in the chronic physiological control of FSH and LH in adult men, these data indicate that (1) testosterone alone, as an androgen, has little effect on FSH or LH, (2) estradiol (or total estrogen) has a greater suppressive effect on FSH than on LH and by its effect on FSH may indirectly regulate the secretion of testosterone, and (3) testosterone and estradiol together may be involved in the regulation of both FSH and LH.