Progesterone is detectable in the peripheral circulation of lactating monkeys for several weeks following parturition, but rapidly falls to undetectable levels in postpartum monkeys not allowed to suckle their young. At laparotomy on the 25th postpartum day, unexpectedly large corpora lutea were found and were subsequently shown by carbon marking to be the corpora lutea of pregnancy. In lactating animals these corpora lutea had the histological appearance of active glands and secreted quantities of progesterone into the ovarian vein approximating those observed during the midluteal phase of the cycle. The nonlactating, postpartum animals, on the other hand, had involuted corpora lutea and lower concentrations of progesterone in ovarian vein blood. The postpartum corpora lutea, in contrast to those of the cycle, secreted more estrone than estradiol into the ovarian venous plasma. It is concluded that suckling prolongs the morphological and functional integrity of the postpartum corpus luteum in primates. (Endocrinology93: 954, 1973)