Ultrastructural morphology of the rabbit aortic intima is described for areas demarcated or not demarcated by spontaneousin vivouptake of the azo dye Evans Blue. Intimal foci taking up Evans Blue are characterized by endothelial and subendothelial cells showing various degrees of degenerative changes, appearance of young cells and subendothelial fibromuscular thickening. It is concluded that dye uptake is a marker for regions subjected to hemodynamic ‘stress’ due to turbulence or increased flow load, and that the morphologic changes in normal animals are part of appropriate regeneration processes which keep the artery functionally intact. Such foci may represent predilection areas for atherogenesis in pathological situations, such as hypertension or hypercholesterole