The morphological features, surface charge and fluid properties of the membranes of aortic endothelial cells of young and old Wistar rats were studied by electron microscopy. Remarkable structural changes in the endothelial cells and subendothelial layer were observed in the aorta of old rats, due to accumulation of fibrillar and granular substances and cellular debris. Analysis of the possible age-related alterations in the permeability properties of the aortic endothelium indicated that positively charged macromolecules such as cationized ferritin are more rapidly transported through the endothelium of old rat aorta, as compared to young rats, thereby leading to their accumulation in the aortic intima of the old animal. Cytochemical analyses of the charge properties of endothelial cell membranes revealed that despite the general similarity in the anionic site density and distribution, endothelial cells from old rat aorta are characterized by a significant decrease in the density of cell-surface sialic acids. The ability of anionic sites in the luminal front of the endothelial cells to redistribute reversibly by lateral migration under the influence of multivalent ligands was shown to be an important factor in the mechanism of internalization and transport of cationized ferritin through the endothelial barrier. It was therefore suggested that the local changes in the surface charge properties due to an interaction with positively charged substances, may account for the accumulation of plasma components in the blood vessel intima of the old animals.