AbstractIt is known that the distribution of stress and strain in the vessel wall is not uniform. We believe that this explains the location of the plane of dissection in dissecting aneurysms of large elastic arteries. We have investigated the effects of non‐uniformity of stress and strain on the thickness of each elastic lamella and on the distribution of intercellular junctions in the media of developing and adult rats, to seek evidence to support this hypothesis. Intercellular junctions were identified by transmission electron microscopy of whole wail sections. A morphomeiric study of elastic tissue distribution was made on an image analysis computer. Differences were analysed using one‐way analysis of variance.There are between six and eight elastic lamellae in the aorta of rats. In the fetus, only the internal elastic lamella is complete; the others were not fully formed by term. In the adult, the inner five elastic lamellae were thicker than the remaining two or three, and smooth muscle cells in the thicker lamellar units had more cell‐cell contacts of all types examined. These data support the concept of a difference in stress‐resisting properties of the aortic wall on the junctions between the inner two‐thirds and the outer third of the media.The findings indicate that, as proposed in theoretical models the innermost lamellae support the high tension. In the adult aorta, the structure is modified to enhance the capacity to resist stress in the internal two‐thirds of the media. This leaves a clear change in form at the junction with the external third of media, where tearing might be expected with exposure to high levels of stress. We suggest that this is critical in the location of aortic