Whole or half placentas from 12- and 14-day-old rat conceptuses were implanted to the surface of the spleen of adult and immature homologous female recipients. Trophoblastic giant cells of the implanted placentas persisted to an age of 20–22 days, a time roughly equivalent to the length of gestation. Placental labyrinth persisted to an age of up to 31 days in implants made to adult recipients but not in those made to weanlings. Continuous daily injection of 2 mg progesterone into immature hosts failed to prolong the persistence of trophoblastic giant cells but improved the appearance of persisting labyrinthine cytotrophoblast. Evidence is presented to show that placental implants may cause decreases in ovarian weight of immature recipients, and that they may prolong the lives of adult recipients that have been bilaterally adrenalectomized and given no supportive therapy. (Endocrinology76: 1068, 1965)