GnRH double stimulation (2 X 25 micrograms i.v. at a two-hour interval) was used to assess the dynamics of LH and FSH release in 25 healthy women on oral contraceptives, all containing 50 micrograms of ethinylestradiol (EE). The study included two sequential regimens (50 micrograms EE X 7 days, 50 micrograms EE + 0.125 mg desogestrel X 15 days; 50 micrograms EE X 7 days, 50 micrograms EE + 2.5 mg lynestrenol X 15 days) and three combined preparations (biphasic: 50 micrograms EE + 1 mg norethisterone acetate (NETA) X 11 days, 50 micrograms EE + 2 mg NETA X 10 days; monophasic: 50 micrograms EE + 2 mg cyproterone acetate X 21 days; 50 micrograms EE + 2.5 mg lynestrenol X 21 days). The tests were always performed on days 19 to 21 of the first treatment cycle and compared to results obtained in normally cycling controls and in women receiving 50 micrograms EE daily alone. It was found that the EE-induced augmentation of pituitary responsiveness to GnRH is diminished by the addition of progestins. LH and FSH reactions to stimulation were both affected. The degree of inhibition depended not only on the chemical structure and daily dose of the progestin component, but also on the duration of its administration per treatment cycle.