IN EARLIER communications we have described a rapid pregnancy test which is suitable for the determination of pregnancy in humans, with an accuracy of 99% within a test period of 8–24 hours (Zondek, Sulman, and Black, 1945; Zondek & Sulman, 1947). The test animal employed in this technic is the infantile female rat, in which species an injection of pregnancy urine elicits the anterior pituitary reaction II (APR II) after a period of 2 hours or more. Marked ovarian hyperemia distinguishes APR in the rat, which differs in kind from APR II in the mouse and rabbit, where it is characterized by follicle hemorrhage (blood points, Zondek and Aschheim, 1926, 1927). The use of the 24 hr. hyperemia reaction as a routine test of pregnancy in equids has been described (Zondek and Sulman, 1945). It was important to determine whether pregnant mare’s blood gonadotropin (PMBG) is similar to human pregnancy urine gonadotropin (PU) in its ability to evoke the hyperemia reaction within a few hours, when minimal quantities of serum are injected. Clarification of this point was of particular interest since the gonadotropin of pregnant mare’s blood consists largely of follicle stimulating hormone (FSH), whereas human pregnancy urine contains chiefly luteinizing hormone (LH). It has been found that the hyperemia test is highly sensitive to PU, but much less sensitive to PMBG.