The microvascular architecture of the pig placenta was studied by serial semithin histological sections for light microscopy, which were compared with scanning electron microscopy of artificially exposed materno-fetal contact surfaces as well as of vessel casts prepared from the maternal, fetal, and combined maternal and fetal sides. The superficial reliefs from the exposed surfaces as well as from the casts are almost identical with the complementary maternal and fetal sides. In order to meet the physiological needs of materno-fetal exchange for the rapidly growing fetuses, these reliefs develop from a simple to a more complex system during pregnancy and can be described as follows: (1) The degree of interlocking increases between the fetal ridges or bulbous protrusions and maternal ridges of different orders separated by maternal troughs of variable depth, most clearly seen on vessel casts. It creates a three-dimensional notch-arrangement, giving strength to the materno-fetal contact area. (2) The structure of precapillary vessels as well as of the meshwork, and the diameter of capillaries of the maternal and fetal sides, adapt during gestation giving a good distribution of oxygenated blood into the maternal capillaries; these, with the development of large prevenous connecting capillaries on the fetal side, favour a high arterio-venous difference of fetal blood O2 pressure. (3) The vascular architecture of endometrial and fetal ridges and troughs develop into a crosscurrent to countercurrent materno-fetal blood interrelationship. Our demonstration of the materno-fetal capillary inter-relationship in the porcine placenta thus shows that the latter is a much more efficient organ for exchange than hitherto assumed.