There are many convincing arguments to accept the existence of inhibin. This hormone is produced inside the seminiferous tubules by the Sertoli cells in males and by the granulosa cells of the follicule in females. The biological, immunological and chemical characteristics of testicular and ovarian inhibin are identical so that it could be speculated the same molecule is secreted by both organs. This hormone is not a knownsteroid but is a protein substance. Thus, its biological activity is destroyed by trypsin and pepsin digestion and by heating at 60 degrees for 30 minutes. Furthermore, immunization with inhibin from rete testis fluid induces antibodies capable of neutralizing endogenous inhibin of adult male and female rats. This polypeptide hormone is not identical neither to ABP nor to a fragment of gonadotrophins. The molecular weight is not yet exactly defined and the possibility exists that two forms of inhibin are present in RTF: one of high (greater than 10,000 Daltons) and the other of low molecular weight. The high M.W. species could be a polymer or alternatively the combination of native inhibin and a carrier substance or unique precursor molecule. Inhibin preparations selectively depress the synthesis and the release of FSH in pituitary cell culture. The threshold dose to affect the LH production is higher than that active on FSH secretion. Furthermore, they reduce LH-RH content of hypothalamus maintained in organ culture. In animals, inhibin induced effects are depending on both hypothalamus and pituitary actions according to the functions of these two structures. In that sense, apparently contradictory results are obtained in short and long term castrated animals. Inhibin does not modify TSH, GH and prolactin in vivo and in vitro. This substance displays an inhibition on the synthesis of DNA in the testis of pubertal male rats and depresses the maturation of follicle in female.