Arteriosclerosis is the consequence of numerous factors which may be studied by inspecting; the composition of blood (cholesterol) and haemostasis, as well as, modifications in the composition and structure of aging arterial-wall tissue. Arterial-tunica cells synthesise in particular, elastin and proteoglycan (mucopolysaccharide-protein complexes) which account for the artery's mechanical and elastic properties. We have shown by biochemical separation and characterization that proteoglycan synthesis is considerably modified in aging arterial tissue. The metabolism of these macromolecules declines. Large aggregates of proteoglycan form. Alterations are observed in the distribution of the different protein-associated long chain polysaccharides, in fatty acids and in calcium bound to these proteoglycans favorising the formation of arteriosclerotic plaques. Studies on the aging of arterial cells in culture also demonstrate these same alterations in proteoglycans.