The segmental microscopic architecture of the aortic wall of guinea pigs and albino rats was studied at the thoracic ascending (T2-3), thoracic descending (T6-7) and abdominal infrarenal (L5-6) levels. Variables such as layer thickness, the number of elastic lamellae in the medial layer, and the diameter of the aortic segments were analyzed histomorphometrically. The aortic wall of both species showed the usual elastic pattern, although variable amounts of elastic lamellae, collagen fibers and smooth muscle cells were observed in the segmental analyses. A marked reduction in the number of elastic lamellae in the medial layer and in vascular diameter was observed in the abdominal aortic segment of both species. Intimal folds, a gradual decrease in elastic lamellae from the ascending to the aorta abdominal aorta and a meshwork of adventitial collagen fibers and elastic lamellae were observed. These data indicate that there are small but significant segmental variations in the aortic wall of these two species.