Abstract: In the present study, the antipregnancy effect of( D-Ala⁶, Pro⁹-Ethylamide~(10))LHRH (LHRH-A)on midgestational rats was observed The results indicate that LHRH-A could terminate pregnancy of all experimented rats by abortion after subcutaneous administration of 200μg LHRH-A(dissolved in vegetable oil)once a day at days 9 to 11 of their pregnancy. It was also found that the serum progesterone level was significantly lowered after the administration ofLHRH-A (P<0.05);the concomitant administration of megestrol acetate with LHRH'-A could almost completely block the antipregnancy effect of LHRH-A on the midgestational rats; LHRH-A showed a strong direct inhibitory effect on luteal cell progesterone secretion in pseudopregnant and midgestational rats. Hence, it is considered that the probable mechanisms for the antipregnancy effect ofLHRH-A may be mainly due to the decrease of serum progesterone level caused by the direct inhibitory effect of LHRH-A on midgestational corpus luteum progesterone production.