Effects of RMI 12,936, a synthetic antiprogestational steroid, on the oestrous cycle and ovulation in the rat.

RMI 12,936 inhibited the vaginal cycle and ovulation in the rat. This effect was not mimicked by oestrogen and was partly reversed by progesterone. Ovulation was restored by injection of hCG and the inhibition was associated with reduced cyclic and tonic LH secretion while hypophysial LH levels were generally unaffected. Hypophysial sensitivity to LH-RH was reduced compared with that when ovulation was blocked with sodium pentobarbitone. It is concluded that RMI 12,936 blocks ovulation by causing a reduction in hypophysial sensitivity to LH-RH and that this is probably an antiprogestational effect.