It has previously been reported that subdermal implants containing d-norgestrel inhibit ovulation through blockage of the positive feedback action of estrogen on luteinizing hormone (LH) release. In this study 100 micrograms of LH-releasing hormone (LRH) were injected intravenously to test the pituitary reserve capacity for gonadotrophin secretion in three women with three 40 mg d-norgestrel rods implanted subdermally for more than one year. The gonadotrophin release to LRH varied and seemed related to the ovarian steroid concentrations at the time of the LRH infusion in a manner similar to that seen during the normal menstrual cycle. It is concluded that low plasma levels of d-norgestreol do not inhibit the pituitary responsiveness to LRH. The results indicate that the blocking action of the gestagen on the positive feedback of estradiol on LH release occurs at the hypothalamus or higher CNS centers.