Blockade of ovulation in rabbits by RU-486, a competitive progesterone antagonist

The effect of an anti-progesterone compound, RU-486, administration (20-150 mg) on ovulation in rabbits was investigated. All of the seven control oil-treated rabbits ovulated, whereas, ovulation was blocked in one (12.5%) of eight rabbits treated with 20 mg RU-486, and in 4-6 (66.7-85.7%) of the 6 or 7 rabbits treated with 40-150 mg RU-486. Two successive injections of 10 mg progesterone (P) 4 and 8 hr after RU-486 injection did however recover ovulation in 3 (75.0%) out of 4 rabbits. These results indicate that P secreted from mature follicles during the ovulatory LH surge also play a significant role in follicular rupture of the rabbit, a copulatory ovulator.