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Inhibin extracted from human seminal plasma and from rete testis fluid is a protein substance. Two forms of inhibin exist in RTF and each has a different molecular weight. The species of higher molecular weight systematically yields a product of lower molecular weight during chromatography. This phenomenon could represent depolymerization of the molecule, an associated transport protein or alternatively a precursor form of inhibin. The inhibin preparations utilized selectively lowered the levels of FSH as assessed both in vivo and in vitro. This action was not totally specific since increasing doses of inhibin also produced a lowering of the levels of LH. In contrast, these preparations did not influence the secretion of TSH, growth hormone or prolactin in vivo or in vitro. A direct effect of inhibin on pituitary cells has been clearly established by demonstrating a reduction in the release of FSH in response to GnRH and its synthesis in the pituitary cells. Additional direct effects on the hypothalamus and on gametogenesis in the testis remain to be excluded. On the basis of selective effects of inhibin on FSH secretion, one could envisage the utilization of this hormone as an antifertility agent.

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