BLOCKADE OF OVULATION IN RABBITS BY RU-486, A COMPETITIVE PROGESTERONE ANTAGONIST

The effect of an anti-progesterone compound, RU-486, administration (2 0-150 mg) on ovulation in rabbits was investigated. All of the seven c ontrol oil-treated rabbits ovulated, whereas, ovulation was blocked in one (12.5%) of eight rabbits treated with 20 mg RU-486, and in 46 (66 .7-85.7%) of the 6 or 7 rabbits treated with 40-150 mg RU-486. Two suc cessive injections of 10 mg progesterone (P) 4 and 8 hr after RU-486 i njection did however recover ovulation in 3 (75.0%) out of 4 rabbits. These results indicate that P secreted from mature follicles during th e ovulatory LH surge also play a significant role in follicular ruptur e of the rabbit, a copulatory ovulator.