The mouse placenta consists of fetal blood vessels, interpolating trophoblast cells and maternal blood spaces forming a labyrinth. It was observed that in contrast to the rapid growth of fetuses, the placentae maintained a constant size through pregnancy. The weight ratio of fetus/placenta was 30:1 at birth about 5 times that in humans. To investigate the efficiency of the labyrinth in materno-fetal exchange in maturing placenta, we histologically examined the formation of the labyrinth as pregnancy advanced. In the late stage, fetal blood vessels and maternal blood spaces had fine, close-knit branches that formed a complex labyrinth, which may explain the efficient exchange of nutrients and wastes between fetus and mother. The maturation of mouse placenta is accompanied by the formation of a more complex labyrinth rather than on any increase in size.