Hamsters injected with 60 i.u. pregnant mares' serum (pms) on the morning of metoestrus (Day 1 of oestrous cycle) will ovulate sixty to seventy ova if they are isolated from males. However, if similarly treated females are caged with males on Day 2 or 3, coitus takes place and the inactive corpora lutea of the oestrous cycle are transformed into functional corpora lutea of pseudopregnancy. Consequently, the animals do not ovulate. The corpora lutea block ovulation by producing progesterone, which inhibits the release of pituitary luteinizing hormone (lh). The corpus luteum of ovulation thus can be brought to full secretory activity as late as 54 hr after ovulation. Pregnant mares' serum-treated hamsters mating late on Day 3 do ovulate presumably because of regression of the corpora lutea. Hamsters placed with males on Day 4 mate at the normal time and ovulate as many ova as isolated controls. Animals injected concurrently with 60 i.u. pms and 5 mg of progesterone on Day 1 and subsequently isolated from males fail to ovulate, thus duplicating the effects of coitus. This experiment also indicates that the ovulation of follicles matured by pms is due to endogenous gonadotrophins rather than the inherent lh activity of the pms preparation.