The histological, ultrastructural, morphometrical and histochemical aspects of the arterial media were studied in young and aged SHR, and compared to normotensive Wistar Kyoto rats. The diffuse thickening was the most characteristic feature of the hypertensive media. It seems due to three processes: Early generalized hypertrophy of smooth muscle cells (smc); connective matrix neogenesis and smc proliferation, more evident in peripheral vasculature. The present paper discusses the following hypertensive tunica media changes in relation to the atherosclerotic process: the decrease in lipolytic esterase and cholinesterase activities; the activation of some lysosomal enzymes; the increase in collagen, glycosaminoglycan and elastin content; the increased media thickness; the modified smc behavior (migration, secretion, proliferation). These alterations might positively influence arterial susceptibility to atherosclerosis through reduced smc lipolytic activity; slowed transmural diffusion; perturbed efflux and aggravated media hypoxia.