SIXTY years ago, Cole and Hart (1) reported that serum taken from 62 mares at various stages of pregnancy and injected into immature rats and mice stimulated ovarian growth. Serum collected from mares before day 37 of pregnancy had no effect, while serum collected between days 37 and 42 induced a pronounced gonadal response. Maximal stimulation was produced by serum taken between days 43 and 80 of mare gestation, and effects could be induced by serum collected through day 131. There was little activity for the remaining 210 days of gestation. The element that stimulated the ovaries became known as PMSG. It was first assumed to be produced by the equine pituitary gland, but its localization in fetal and maternal tissues led Catchpole and Lyons (2) to conclude that it was secreted by the chorion and stored in the endometrium. It was later shown that the site of synthesis of this gonadotropin was the endometrial cups (3, 4), which are pale, circumscribed plaques of tissue that develop in the gravid horn of the mare endometrium, adjacent to the chorionic girdle of the embryo (5).