In order to assess the possible role of circulating immunoreactive inhibin (INH) during the menopausal transition, two groups of subjects were studied. Four were normal volunteers, three of whom had developed their first symptoms of cycle irregularity at age 45-46 years, the fourth being aged 37, a volunteer for a study involving daily blood sampling found to have a transient rise in serum follicle stimulating hormone (FSH). Six were patients with anovulatory infertility, aged 34-44 years, found to have transitory ovarian failure during attempts at ovulation induction. Intermittent blood samples were obtained for radioimmunoassay of serum FSH, luteinizing hormone (LH), INH, oestradiol (E2), and progesterone. Abrupt changes were observed, with transient elevations of FSH and LH and decreases of INH and E2 into the postmenopausal range, followed by levels more characteristic of reproductive-aged women. It was concluded that typical postmenopausal hormone patterns may occur at the time of entry into the normal menopausal transition, and in some women with anovulatory infertility, but may be completely and relatively abruptly reversible. Elevation of serum FSH into the postmenopausal range, with undetectable INH concentrations, does not provide reliable evidence that the menopause (or permanent ovarian failure) has occurred. INH contributes to elevations of serum FSH during the menopausal transition.