The numbers of plasmalemmal vesicles in endothelial cells of rat blood vessels were determined on electron microscopic sections. In all vessels examined which included aorta and carotid and femoral arteries, vena cava and femoral vein, and lung and brain capillaries, the numbers were of the same order of magnitude. For arteries the numbers were about double those for the corresponding veins. About one-third of all vesicles could be stained with ruthenium red after its infusion into the vessels. The results make it improbable that differences in numbers of `transport' vesicles in different types of blood vessel contribute significantly to the selective accumulation of atherogenic plasma proteins in arteries.