During the luteal phase of the menstrual cycle, plasma progesterone (P) and estradiol (E2) concentrations are elevated, and LH (and by inference GnRH) pulse frequency is slow. In contrast, LH pulse frequency increases during the early follicular phase when plasma E2 and P are lower. To examine the mechanism(s) responsible for the slower GnRH pulse frequency in the luteal phase, we maintained plasma P, E2, or both at midluteal concentrations from the midluteal phase to the time of the next early follicular phase and measured the effects on LH secretion. Thirteen normal women with regular menstrual cycles were studied during two or three cycles. Blood was obtained every 10 min during 10-h studies. Control cycle luteal and early follicular studies were followed by a second control study in the luteal phase of the treatment cycle. P (six women), E2 (seven women), or both (five women) then were given twice daily by im injection for 6-12 days until the day corresponding to the early follicular study of the control cycle (EF + P, EF + E2, or EF + E2 + P). A final study was performed 1 week after the injections were discontinued (F). LH pulse frequency was low in the midluteal phase [3.2 +/- 0.2 (+/- SE) pulses/10 h] and increased by the early follicular phase (8.0 +/- 0.8 pulses/10 h) in the control cycles. The increase in LH pulse frequency was not significantly inhibited by administration of P (6.7 +/- 0.7 pulses/10 h; EF + P). However, during both E2 alone and E2 + P, LH pulse frequency remained low (EF + E2, 3.6 +/- 0.8; EF + E2 + P, 2.0 +/- 0.7 pulses/10 h). The mean plasma FSH concentrations paralleled changes in LH pulse frequency, increasing from the luteal to the early follicular phase in the control cycles and during P injections and remaining low during E2 and E2 + P injections. We conclude that continued exposure to P alone does not maintain GnRH pulse frequency at midluteal phase values and that any effect of P requires the presence of E2. As E2 alone maintained lower LH pulse frequency, E2 may act directly to decrease the pulsatile GnRH secretion or it may potentiate the effects of low (less than 3.2 nmol/L) P concentrations.