Eleven regularly menstruating women between 22 and 35 years of age with a usual cycle length of 27 to 31 days volunteered for this Study. Volunteers received an intra-venous injection of 100 µg synthetic LH-RH and 200 µg synthetic T-RH during the early and the late follicular phases and during the early and midluteal phases of the menstrual cycle.

LH-RH induced a prompt increase in circulating LH, reaching the peak concentration at 30 minutes following LH-RH administration in all phases of the cycle studied. A change in responsiveness with greater and more sustained LH release from the early to the late follicular phases was observed.

The response during the luteal phase was significantly greater than the responses in both the early and the late follicular phases. A concomitant but a much smaller FSH response was observed. T-RH elicited a prompt increase in circulating prolactin within 30 minutes and decreased gradually thereafter, reaching the baseline level by 2 hours after T-RH administration. Maximum concentration of prolactin was reached in 30 minutes following T-RH during all phases of the menstrual cycle. No variation in pituitary responsiveness to T-RH, however, was observed during different phases of the menstrual cycle.

These data indicate that the sensitivity of the pituitary gonadotrophs to LH-RH varies during different phases of the menstrual cycle.