On the basis of anatomical and histologic studies, four distinct zones of the aorta of the atherosclerosis-prone White Carneau pigeon were defined. From its origin to about 1 cm. above the level of the celiac artery (the elastic zone), the aorta had a media composed of alternating lamellae of smooth muscle cells and fibroblasts with elastic plates between them. Distal to the elastic zone was the transitional zone, which was about 1 cm. long and ended at the level of the celiac artery. At this level there was an intimal cushion of longitudinally arranged smooth muscle cells that was present at birth but increased in thickness with age; an occasional lymphocyte-like cell was also present in the cushion. Typically, the atherosclerotic lesion originated in the smooth muscle cell cushion. In the transitional zone the media had an elastic architecture in the lateral third, the remainder being predominantly muscular. The segment of aorta from the celiac artery to the ischiadic arteries (hybrid zone) had a hybrid architecture with a thick muscular anterior wall and a thin elastic posterior wall. The structure distal to the ischiadic arteries was purely muscular (muscular zone). The presence of intimal smooth muscle cells in the transitional zone at birth, in the precise location where the "classic" atherosclerotic lesion develops months later, suggests that atherosclerosis develops within preexisting smooth muscle cell cushions which apparently represent a normal histologic feature of this area.