Ovarian estrogen secretion stimulates the hypothalamo-pituitary axis of the rat to release an ovulation-inducing surge of luteinizing hormone (LH) with the potential for continued release every 24-h. The primary objective of this proposed project is to characterize the mechanism through which the LH surge is inhibited. This will be accomplished by: (1) determining the sites within the hypothalamo-pituitary axis at which steroids act to inhibit the preovulatory release of LH, (2) defining the role of extrahypothalamic neural structures in the inhibition of the LH surge and (3) determining if LH-release-inhibiting activity exists within the hypothalamus and if its activity correlates with known states of inhibited LH release during the estrous cycle. The results of these studies should provide an understanding of the physiological mechanisms whereby the preovulatory release of LH is inhibited and thus may suggest alternative approaches to the control of fertility.