Inhibin is a dimeric glycoprotein hormone comprised of an α and ß subunit. It is a member of the transforming growth factor-ß (TGF-ß) superfamily and inhibits the production or secretion of pituitary gonadotropins, preferentially follicle-stimulating hormone (FSH). Inhibin with activin, a closely related dimeric glycoprotein hormone comprised of two ß-subunits, create a fine-tuned endocrine feedback loop. Inhibin decreases, while activin increases, the biosynthesis and release of FSH. Inhibin and activin have been demonstrated to be present in a variety of gonadal and nongonadal tissues, indicating that these peptides have other functions in addition to regulating FSH secretion. Inhibin antagonizes the action of activin in many systems, which may be a property valid in tumorigenesis. It is also thought that inhibin may act as a gonadal tumor suppressor, while activin may promote tumor growth via an autocrine loop.