The threshold of sensitivity of the gonadostat has been tested in 20 old menopausal women by clomiphene (50 or 200 mg/day for 5 days) and/or low doses of ethinyl oestradiol (EE2) (10 or 20 microgram/day). Clomiphene did not increase nor decreased serum levels of LH-FSH and testosterone (T) but increased significantly androstenedione (A) levels. Even at the lower dosage used, EE2 decreased drastically the levels of LH but less completely that of FSH. Clomiphene breaks off partially the oestrogen's blockade on FSH but not on LH. No modification was observed for T and A even after 30 days of estrogen therapy. This data shows the peculiar sensitivity of the gonadostat in advanced menopaused women. It also suggests that the gonadostat exhibits a low threshold of sensitivity analogous to that observed in late prepubertal period. Moreover, the different behaviour of LH and FSH in response to the negative feedback of oestrogens supports that an ovarian factor, distinct from estrogens and missing during menopause plays a role in the regulation of FSH.