In Vivo: [D-Phe2,D-Ala6]-LH-RH inhibits the LH and FSH release induced by LH-RH in immature male rats. [D-Phe2,D-Ala6]-LH-RH (1 mg/rat, SC) shows marked antiovulatory and contraceptive activity.[D-Phe2,D-Ala6]-LH-RH (0.625, 1.25, 2.5 mg/kg; s.c.; single dose; sacrificed 30 minutes) results a dose-dependent increase in stalk-median eminence (SME)-LHRH activity and decreases plasma LHRH activity as well in rats with hypophysectomized (hypox) at 25 days old. [D-Phe2,D-Ala6]-LH-RH (25 mg/kg; s.c.; single dsoe) is effective in blocking the ovulation normally induced by LRH alone.