Stalk-medium eminence (SME) and plasma LHRH activities of chronically hypophysectomized immature female rats were evaluated by bioassay following a single subcutaneous injection of varying doses of either LHRH or the LHRH antagonist, [D-Phe2, D-Ala6]-LHRH (Wy-18,185). Administration of LHRH to hypophysectomized rats produced parallel dose-related increases in SME and plasma LHRH activities. Wy-18,185 produced a similar, but attenuated dose-related increase in SME-LHRH activity, while a dose-related decrease in plasma LHRH activity was observed. Collectively, the data suggest that (1) exogenous LHRH may be sequestered by the hypothalamus leading to significant increases in hypothalamic LHRH content, and (2) peptide antagonists of LHRH inhibit the release of endogenous LHRH at the hypothalamic level.