Previous studies on cholesterol-induced sudanophilic lesions in rabbit aortas showed that the earliest lesions were periorificial with the greatest involvement distal to branch sites. In order to determine if there is a relationship between endothelial cell morphology and the regional differences in susceptibility to atherosclerosis we compared cell morphology distal to several aortic ostia and away from ostia. We examined the en face morphology of the intimal surface. Vascular casts were made of silver stained normal aortae. The surface was micrographed by SEM. Cell outlines were entered into a computer to calculate three parameters: the cell orientation index which indicates the cell shape, the en face cell area, and the distance between centers index which gives the position of the center of area with respect to the center of a longitudinal axis drawn through the cell. Differences were found between the region distal to branch sites and the region away from branch sites. Differences were also found between the regions distal to different branch sites. The complex variation of morphology led to no conclusive relationship between endothelial cell morphology and atherogenesis in the non-cholesterol-fed rabbit.