Small-angle X-ray diffraction patterns of bovine, human and porcine media aortic wall show meridional reflections corresponding to a periodicity which suggest a molecular packing typical of tendon collagen. However the meridional intensity distribution of stretched air dried aortic samples appears different from that of air-dried tendon, probably because of the presence of a large amount of type III collagen with the environment, which are specific for aortic tissue. The stretched wet aortic samples show a marked decrease in intensity, revealing an extensive disorder in the axially-projected structure of the fibrils. When a loading system simulating the effect of blood pressure is applied to a ring of aorta, no evidence of orientation of collagen is seen by X-ray diffraction, as would be expected if collagen fibrils had an isotropic distribution inside the aorta media. Scanning electroni microscopy supports the existence of a network of collagen fibrils surrounding elastic lamellae.