Inhibin is a glycoprotein which specifically inhibits FSH secretion. In follicular fluid, several forms of inhibin have been identified with mol. wts between 120 and 32 kd. The smallest active form is a glycosylated 32-kd protein with an 18-kd A chain and a 13-kd B chain. The secretion of inhibin by granulosa cells of the follicle is stimulated by FSH and testosterone, but not LH, and treatment of women with clomiphene and HMG/FSH results in an increase in the plasma concentration of inhibin. However, during the follicular phase of the normal menstrual cycle, FSH is inhibited mainly by oestradiol and inhibin levels remain constant or decline in parallel with those of FSH. It is probable that inhibin and oestradiol act together to cause the suppression of FSH. During the luteal phase, the human corpus luteum under the influence of LH secretes inhibin as well as oestradiol and progesterone. This combined negative feedback signal from the corpus luteum accounts for the suppression of FSH release and thus the failure of follicular growth during the luteal phase.