By using scanning electron microscopy (SEM) we were able to follow the sequence of ultrastructural alterations of the luminal surface which occurred when specimens of canine carotid artery were subjected to controlled transmural pressures. Specimens of carotid artery were removed from dogs following fixation at experimental pressures ranging from 0 mm Hg to 100 mm Hg. The endothelium of specimens fixed at 0 mm Hg has parallel longitudinal ridges formed by the contraction of the underlying internal elastic lamina. With increasing transmural pressure, the luminal surface undergoes a gradual flattening of the endothelial ridges so that at 100 mm Hg, these ridges have completely disappeared. The observed morphologic changes of the arterial endothelium indicate that SEM can provide good ultrastructural information on blood vessels subjected to controlled transmural pressure and that the pressure-dependent alterations must be considered in studies on vascular structure and function.