The media of arteries is virtually avascular. Thus, oxygen and nutrients must reach the cells of the media by diffusion from the lumen of the vessel and from adventitial vessels. The thickness of the thoracic aorta of man and dog exceeds the effective diffusion distance of oxygen, but nutrition is supplemented by vasa vasorum which enter into the outer layers of the media. Occlusion of vasa vasorum in dogs produces medial necrosis, which indicates that these vessels are essential for the nourishment of the aorta. Recently the microsphere method has been used to provide the first measurements of blood flow through vasa vasorum. There is substantial flow to the outer layers of media of the thoracic aorta in dogs, with virtually no blood flow in the inner layers. The vessels are very responsive to physiological stimuli: they dilate during infusion of adenosine and constrict during stimulation of sympathetic nerves. During acute increases in arterial pressure, blood flow to the media decreases probably from distortion of vasa vasorum. Vasa vasorum may play a role in disease states. Insufficient blood flow through vasa vasorum may contribute to medial necrosis of the aorta and to aortic atherosclerosis. A role of vasa vasorum in aortic changes in hypertension and other vascular diseases merits further study.