The purpose of this study is to examine the effect of LH-RH on LH release in the baboon. Fifteen female baboons having the normal menstrual cycle were used for this study. On hundred mug of synthetic LH-RH was injected subcutaneously in both the early follicular phase and the early luteal phase. For control purposes, 1 ml of saline was injected subcutaneously in the luteal phase. Blood samples were collected by femoral vein puncture with light anesthesia under prearranged schedule and were assayed for LH-RH, LH, estrogen and progestin. The plasma level of LH-RH reached a maximum within 4 minutes after s.c. injection of 100 mug LH-RH, decreased sharply at first, and then slowly later. Fast and slow disappearance components (t1/2 = 4.7 min., 37.1 min. respectively) were observed. In the baboon given LH-RH during the luteal phase, peaks in plasma levels of LH were observed within 30 minutes and within 90 to 150 minutes after injection. A lesser pituitary response to LH-RH for LH release occurred during the follicular phase. The first peak of LH was well-correlated with the peak of plasma LH-RH but the later elevations of LH (observed within 90 to 150 minutes after LH-RH injection) were not necessarily related to the plasma level of immunoassayable LH-RH. Elevation of plasma levels of estrogen and progestin was observed wtihin 45 minutes after LH-RH injection. In saline control, the plasma level of LH was not elevated; however, plasma levels of estrogen and progestin were increased within 45 minutes after saline injection. Later elevation of plasma LH observed between 90 and 150 minutes after LH-RH injection may be due to administered LH-RH in cooperation with elevated levels of plasma estrogen and progestin. To pursue this problem, injections of estrogen and/or progesterone were performed during the early follicular phase. Injection of 600 mug of estrodiol benzoate (EB) for 3 days caused an elevation of plasma level of LH and enhanced pituitary LH responsiveness to LH-RH for LH release; however, injection of 100 mug EB for 3 days showed less effect on LH release. Injection of 10 mg of progesterone for 3 days also caused an elevation of plasma level of LH and enhanced the pituitary responsiveness to LH-RH release. Injection of both 600 mug EB and 10 mg progesterone for 3 days did not elevate plasma level of LH and showed no significant effect of LH-RH on LH release as compared to control. These results suggest that elevated levels of circulating estrogen and progestin may determine LH release and exposure of the pituitary to LH-RH is necessary for LH release. In dose and time schedule used in this study, it is inferred that estrogen and progesterone may exert their direct effect to hypothalamus on endogenous LH-RH secretion and also may exert their effect on pituitary gonadotrophs to change the sensitivity to LH-RH, i.e. these steroid hormones may be major factors in the control of gonadotropin release in the baboon.