The Placenta plays a pivotal role in the maternofetal exchange of substances. In the chorioallantoic placentas of rodents, maternal and fetal bloods come close in the labyrinth. Hemotrichorial placenta is formed in most of rodents including rat, mouse, and hamster. Three trophoblast layers composed of a single cytotrophoblast layer and two syncytiotrophoblast layers and endothelium of fetal capillaries lie between maternal and fetal circulations in the labyrinthine wall. Many gap junctions are present between syncytiotrophoblast layers. The double-syncytiotrophoblast layers connected by gap junctions serve as a structural basis of the placental barrier as well as a site of specific transfer of various substances. In the guinea pig placenta, only a single syncytiotrophoblast layer is formed, thereby it is classified as a hemomonochorial placenta. Endotheliochorial placenta was reported in the kangaroo rat. The ultrastructural features of the labyrinthine wall are described and discussed in relation to specific transplacental transfer of substances, especially glucose.