The effect of antiserum to progesterone on follicular rupture during the ovulatory process was studied. Mean number of ova shed following treatment of immature rats sequentially with PMS and hCG was reduced in a dose-dependent manner by simultaneous injection with increasing doses of antiserum to progesterone. When the animal received 1.2 ml of the antiserum, hCG-induced ovulation was blocked completely. To be effective, antiserum treatment had to be within 6 h of hCG treatment; antiserum given 9 h after hCG was ineffective. Progesterone restored' the antiserum blocked ovulation completely or incompletely when administered intravenously within 6 h of treatment with hCG. The first 6 h was shown to be a progesterone-dependent step in the ovulatory process in this experimental system.