Ganirelix is a gonadotropin-releasing hormone receptor (GNRHR) antagonist (IC50 = 3.6 nM; pA2 = 9.3). It induces a concentration-dependent increase in histamine release from rat peritoneal mast cells in vitro (EC50 = 11 μg/ml). In vivo, ganirelix (2 mg/kg, s.c.) decreases plasma testosterone in intact male rats for the first 7 days post-administration. Ganirelix (125 μg per day for 30 days) decreases the surface area of endometriotic lesions and serum progesterone levels in female baboons. Formulations containing ganirelix have been used to prevent premature ovulation in women undergoing in vitro fertilization.