Silastic capsules containing estradiol– 17β were implanted into normally cycling rhesus monkeys 2–3 days following the preovulatory LH surge. The resultant plasma estrogen concentrations, which were maintained throughout the luteal phase of the cycle, were approximately one—half those observed on the day of the LH peak but exceeded normal, luteal phase levels by about 150 pg/ml. This increment in circulating estrogen levels was associated with a premature decline in plasma progesterone concentrations which became undetectable 6.4 days before those in control animals, a 43% reduction in the functional life—span of the corpus luteum. These findings demonstrate that small increments in circulating estradiol can induce functional luteolysis in the rhesus monkey and support the hypothesis that estrogens play a role in the physiological regression of the corpus luteum during the normal menstrual cycle of primates. This luteolytic action of estradiol could not always be associated with measurable decrements in plasma LH concentration. (Endocrinology92: 1148, 1973)