PIP: Following the total removal of luteal tissue, circulating plasma progesterone, intrauterine pressure, oxytocin response, and clinical progress in abortion were determined sequentially in 12 first-trimester pregnant patients. 3 patients were ovariectomized for the removal of ovarian cysts and 9 were luteectomized during tubal ligation in an attempt to terminate pregnancy in spontaneous abortion. 7 patients had corpora lutea which averaged 21 plus or minus 1mm in diameter at operation when performed at Day 49 plus or minus 2 (mean S.E.) of pregnancy. These patients responded to ovariectomy or luteectomy by a continuing decrease in progesterone, evolution in intrauterine pressure, oxytocin response, progress in cervical dilitation and, abortion. Abortion occurred with a mean lapse time of 5 plus or minus 1 days after operation. In contrast, 5 patients whose corpora lutea averaged only 11 plus or minus 1mm in diameter when removed at Day 61 plus or minus 4 of pregnancy showed only transient decrease in progesterone after operation. This decrease was followed by an increase in progesterone; no progress in the evolution of intrauterine pressure, oxytocin response, cervical dilatation, and abortion. It appears that so long as the corpus luteum serves as the major source of progesterone, it is indispensable in the maintenance of pregnancy in human subjects as it is in the clinical model animal, the rabbit. However, with the shift of progesterone production from the corpus luteum to the placenta (the luteoplacental shift) the human corpus luteum becomes dispensable. These findings identify the corpus luteum and its secretory product, progesterone, as feasible targets of fertility control strategy.