It has been demonstrated that the ovary bearing the corpus luteum in the human is responsible for the major portion of prostaglandin F2α(PGF2α), total estrogen, and progestin production during the luteal phase of a normal menstrual cycle.1 This study was performed with the intent to gain more information about the secretion of PGF2α in conditions that prolong the life span of the corpus luteum, such as pregnancy and Halban's disease. Utilizing a specific radioimmunoassay for PGF2α, ovarian venous plasma levels were measured in 7 pregnant women and in a patient with Halban's disease. During the first and second trimester of pregnancy, PGF2α values in plasma from the active and inactive ovary were comparable and were significantly lower than concentrations in plasma from the active ovary during the luteal phase of the normal cycle. In a patient with persistent corpus luteum or Halban's disease, PGF2α concentrations of venous plasma from the ovary bearing the corpus luteum were significantly lower than those obtained from the contralateral ovary. These observations indirectly support the hypothesis that prostaglandins produced within the ovary may have a role in luteal regression.