Inhibin is produced by Sertoli cells in the male and by granulosa cells in the female. Follicular-stimulating hormone acts directly to stimulate production whereas luteinizing hormone exerts an indirect effect by stimulating production of androgens which themselves activate synthesis and release of inhibin. Prolactin has no effect on inhibin. These interpretations, derived from numerous in vivo and in vitro studies, explain why inhibin is not secreted in the hypophysectomized animal. Interruption of spermatogenesis by ligature of deferens canals, by experimental cryptorchidism in the rat, and in human primary azoospermias, provokes a reduction in production of inhibin and an increased secretion of FSH. The second must be the consequence of the first. Restoration of normal spermatogenesis results in normal production of inhibin. Biochemical mechanisms linking spermatogenesis and inhibin production are still unknown.