Studies on the secretion of inhibin and its mode of action were carried out in vitro, utilizing cell cultures. Isolated rat Sertoli cells secreted an inhibin-like heat-labile, non-dialysable substance, Sertoli Cell Factor (SCF), which could selectively suppress FSH secretion by rat anterior pituitary cells. SCF selectively suppressed the basal and GnRH-stimulated FSH release as well as the de-novo synthesis of FSH by acting directly on the pituitary cells. In 1 out of 5 experiments, SCF also suppressed the synthesis of LH, possibly by affecting the overall protein synthesis. Under similar culture conditions, Sertoli cells isolated from animals between 18 and 90 days of age secreted comparable amounts of SCF. In contrast, anterior pituitary cells from adult rats (60-90 days old) were considerably more sensitive to SCF than pituitary cells obtained from younger (18-33 days old) animals, suggesting that decline in circulating FSH level, occurring at approximately 35 days of age, may result from increased pituitary sensitivity to inhibin. Besides identifying the Sertoli cells as the site of inhibin production in the testis, these studies demonstrated direct action of inhibin at the pituitary cell level, resulting in suppression of FSH synthesis and release.