Plasma progesterone concentration was determined in the maternal peripheral vein, umbilical vein, uterine vein, and both ovarian veins at elective cesarean section in term pregnancy. In 10 cases plasma from both ovarian veins was analyzed. In all but one the ovarian vein draining blood from the corpus luteum contained greater concentrations of progesterone than did the blood from the side without the corpus luteum. It has been suggested for some time that the corpus luteum of pregnancy continues to function throughout pregnancy. Our data appear to support this concept.