Twelve untreated cycling women and 45 women using oral contraceptives were challenged iv with 50 micrograms of GnRH and 100 micrograms of TRH at 0, 90 and 180 min. The LH responses after the second and third pulses of GnRH/TRH were diminished in women treated with 30 micrograms of ethinylestradiol and levonorgestrel as compared with the control group, whereas the responses were severely blunted after each dose of GnRH/TRH in women treated with 50 micrograms of ethinylestradiol and levonorgestrel or lynestrenol as compared with both the control group and the women treated with 30 micrograms of ethinylestradiol and levonorgestrel. The FSH responses to GnRH/TRH in women treated with 30 micrograms of ethinylestradiol and levonorgestrel were similar to those in the control group, whereas there were reductions in the responses obtained in women treated with 50 micrograms of ehinylestradiol and progestin as compared with both the control group and the women under 30 micrograms of ethinylestradiol and levonorgestrel. In women treated with 30 micrograms of ethinylestradiol and levonorgestrel, the PRL peaks after the second and third pulses were higher than the first, and higher than the values obtained in the remaining three groups.