The three-dimensional architecture of the villous core of the chorionic villi has been studied in the human placenta throughout pregnancy by scanning electron microscopy, and compared with light microscopic and transmission electron microscopic, findings. Five different types of villi are recognizable: the mesenchymal villi are characterized by a network of tiny bundles of collagen fibrils, enmeshing mesenchymal cells, Hofbauer cells and vessels. The immature intermediate villi display a system of intercommunicating channels, parallel to the major axis of the villi. Within the channels, Hofbauer cells are present. The mature intermediate villi show narrow and short stromal channels, surrounded by a rather tight network of collagen fibrils. Large vessels and a highly compact network of collagen are typical of the stem villi. The terminal villi contain sinusoidally dilated capillaries and a small amount of connective tissue. These findings have numerous functional implications. They are particularly related both to the presence of the stromal channels which could facilitate the macrophagic and immunological task of the Hofbauer cells, and to the different types of villi, playing different roles in placental and fetal life, under normal and pathological conditions.