Early dihydrotachysterol-induced calcification of the rat aorta occurs in elastic lamellae. The first deposition of inorganic substance leads to the formation of very thin filament-like structures of low electron density. The characteristic shape of these structures suggests that they could correspond to calcified filamentous components of the elastic tissue. When calcification spreads from the calcified elastic lamellae into the adjacent tissue, the inorganic substance is initially collected in roundish structures, probably of cellular origin, and is successively laid down in the entire matrix of the aortic wall, including collagen fibrils. All the calcified areas contain glycoproteins and acid proteoglycans. A cartilage-like tissue often develops near calcified areas. Its fine structure is very similar to that of the normal hyaline cartilage. It can be calcified, but usually the inorganic substance is not crystalline as it is in normal cartilage. It seems to consist of very small, linearly aggregated inorganic granules which form irregular structures. These seem to develop in close relationship with the fibrillar, probably collagenic, network of the matrix. No ultrastructural findings have been obtained for explaining cartilage induction near calcified areas of the aortic wall. It is possible that cartilage differentiation is regulated by diffusible substances which cannot be recognized under the electron microscope.