To examine the mechanism by which endogenous estrogens inhibit gonadotropin secretion in men, blood samples were drawn every 10 min for 12 h in five men before and at the completion of 3 weeks of treatment with the estrogen antagonist clomiphene citrate (50 mg twice daily). Samples were analyzed for LH and alpha-subunit by RIA. Clomiphene produced a 3-fold rise in circulating LH levels, which was associated with a 80% increase in pulse frequency and a 70% increase in pulse amplitude. Immunoreactive alpha-subunit secretion was also pulsatile before and after clomiphene treatment. Mean alpha-levels rose 70%, together with a 39% increase in pulse frequency and a 41% increase in pulse amplitude. Circulating testosterone and estradiol levels increased 2-fold and FSH levels increased 3-fold after clomiphene treatment. Insofar as each LH and uncombined alpha-subunit pulse reflects a LHRH secretory episode, our data indicate that endogenous estrogens tonically restrain the hypothalamic release of LHRH. From these results and those of previous studies, we conclude that estrogens as well as androgens are important in the testicular feedback inhibition of the hypothalamic oscillator that governs pulsatile gonadotropin secretion.