Inhibin has been defined as a gonadal hormone that exerts a specific negative feedback action on the secretion of FSH from the gonadotropic cells of the pituitary gland. The existence of inhibin was postulated as early as 1923 (250). However, only after reliable and sensitive bioassay systems for the detection and estimation of inhibin had been developed and an ample source of inhibin was found in the form of ovarian follicular fluid has progress been made in the isolation and characterization of the hormone. It is apparent now that inhibin, which itself consists of a dimer of two different subunits, alpha and beta, is a member of a much larger family of (glyco)protein hormones and growth factors, which also includes Mullerian-inhibiting substance, transforming growth factor-beta, erythroid differentiation factor, an insect protein that plays an important role in differentiation, and the dimer of two inhibin beta-subunits, activin. The latter substance was reported to counteract the effects of inhibin in pituitary cells. Interactions between these regulatory substances will certainly be a field of major interest in the near future. Inhibin molecules, similar to those in the gonads, have been detected in placental tissue as well. The postulate of inhibin in seminal plasma, prostate, and gastric juice is apparently due to detection of substances that differ from gonadal inhibin. Most likely these substances, the amino acid sequences of which have been elucidated, have a biological function that differs from the suppression of peripheral levels of FSH in vivo. Inhibin is produced in the Sertoli cells in the testis and in the granulosa cells in the ovary. The production of inhibin is stimulated by FSH, but presently a lot of controversy exists about other factors that might play a role in the regulation of the production of inhibin. Because of the lack of reliable methods for estimation of peripheral levels of inhibin in humans and experimental animals, almost all evidence for the physiological importance of inhibin in the regulation of reproductive processes is derived from circumstantial evidence. From these indirect results, it appears likely that inhibin plays an important role in the feedback regulation of peripheral concentrations of FSH during the period in which Sertoli cells and granulosa cells, the target cells for FSH, divide, i.e., during puberty in male animals and during the development of ovarian follicles in female animals.(ABSTRACT TRUNCATED AT 400 WORDS)