The role of luteinizing hormone (LH) in regulation of secretion of ovarian steroids during the preovnlatory phase of the rat estrous cycle has been discussed. Evidence indicating an involvement of these steroids in control of the uterine growth, vaginal cornification, mating behavior and LH secretion which occur during this phase has been reviewed. A positive feedback control system has been proposed in which both ovarian and adrenal steroids participate to initiate the preovulatory “LH surge” which culminates in ovulation and luteinization