The electron microscope was used to examine the junctional zone of the chorioallantoic placentas of living rat young which had been experimentally retained in utero for as long as 4 days beyond delivery of their littermates. Junctional zone cells in such prolonged placentas were compared with corresponding cellular elements in term placentas. By the 1st day of the postterm period, placental glycogen cells in the junctional zone were depleted. Many trophoblast giant cells and trophospongial elements appeared fully viable throughout the entire 4-day period, however, Giant cells from placentas of prolonged pregnancy were indistinguishable from those in placentas at term. Both in term and in postterm placentas the trophospongial elements displayed a variation in the organization of the granular cisterns of endoplasmic reticulum within them. No variants appearing to prefigure involution were identified either in term or in prolonged placentas. However, more cells with vesiculate cisterns were seen in junctional zones from placentas which were retained beyond term. Intercellular relationships also were preserved during the postterm period. With the integrity of the cells of this zone so well preserved both in term and in prolonged placentas, it is concluded that parturition is not dependent on their involution.