Inhibin is a peptidic gonadal hormone which preferentially suppresses FSH secretion and synthesis. As its purification is not yet achieved, only bioassays are available, performed with testicular extracts or follicular fluid. Its secretion by granulosa cells and Sertoli cells is partly spontaneous, partly stimulated by androgens and FSH. In women, folliculogenesis is controlled by inhibin induced FSH fluctuations. But inhibin acts also at the gonadal level like the cybernins, peptidic substances secreted by the ovary, which modulate ovarian effects of gonadotrophins. Five of them have been identified: the oocyte maturation inhibitor, the luteinization inhibitor, the FSH binding inhibitor (implied in follicular atresia), the LH receptor binding inhibitor (involved in luteolysis), and gonadocrinins (which bind to ovarian LHRH-receptors). The discovery of these ovarian peptids leads to new concepts in folliculogenesis and luteolysis and may provide a new therapeutic approach in contraception and sterility fields.