Mature placental material of full-term spontaneous births and 4-, 6-, 8- and 10-week-old placentas obtained from curettage cases were fixed in glutaraldehyde and osmium tetroxide for SEM examination. In young placentas, the ramifications of the chorionic villi start in the form of buds. The buds are transformed into tendrils with swollen extremities. These swellings resemble buds ready to bloom. The villi intertwine in different positions; both the villi and their tendrils are covered with dense layers of microvilli. In mature placentas, the surfaces of the chorionic villi and their ramifications are covered with microvilli. However, in comparison with the microvilli of young placentas, the microvilli here are less numerous and shorter. Invaginations were clearly visible on the surfaces of the villi; younger and newly budding microvilli, similar to those observed in young placentas, were seen in the invaginated regions. We had the impression that the mature placentas must regenerate in order to meet the increasing physiological requirements of the fetus.