In order to study the action of estrogens and inhibin and their interaction with gonadotropin secretion in men, we measured basal and LRH-stimulated gonadotropin secretion before and after six weeks' administration of tamoxifen in two groups of men. A) Six men with impaired spermatogenesis with basal levels of FSH, LH, testosterone and estradiol within the range of reference values in our laboratory. B) Six men with severely impaired spermatogenesis. LH, testosterone and estradiol levels were within reference values, whereas FSH was at least 100% above the upper limit of reference values. Mean testosterone and estradiol levels were not different in either groups. Inhibin levels were presumed to be normal in group A and decreased in group B. Although within reference values, basal LH levels in group B were slightly but significantly higher than in group A. The response of LH to LRH in group B exceeded that of group A. This suggests that inhibin has a slightly inhibitory effect on basal LH and even more so on LRH-induced LH secretion. After tamoxifen administration basal FSH levels rose far more in group B than in group A. The LRH-induced LH release increased in both groups, the LRH-stimulated FSH release, however, increased only in group A. We conclude that estrogens and inhibin have a strongly synergistic negative feedback action on basal FSH secretion, and to a limited extent on basal LH secretion. In the presumed presence of inhibin (group A) the FSH additionally synthesized through the action of anti-estrogens, can be released by LRH administration, whereas FSH is probably spontaneously released in the presumed deficiency of inhibin (group B). With LH this phenomenon is less obvious.