Vasa vasorum are present in the middle and outer layers of media in the thoracic aorta of dogs and humans. To examine the role of vasa vasorum in nourishment of the aorta, we ligated four contiguous pairs of intercostal arteries in anesthetized dogs. These arteries are the source of vasa to the descending aorta but not the aortic arch. Blood flow through vasa vasorum was measured with microspheres. Acute intercostal ligation did not reduce conductance in the aortic arch but reduced conductance in the middle third of the descending aorta from 7 +/- 1 to 3 +/- 0.7 (SE) ml.min-1.100 g-1.mmHg-1 (P less than 0.05). After intercostal ligation, infusion of adenosine (5 mumol.kg-1.min-1 iv) increased conductance in the aortic arch 3- to 4-fold but did not increase conductance in the descending aorta. Six to ten days after intercostal ligation, conductance in the middle third of the descending aorta remained low. Vasodilator capacity was partially restored in outer layers of the descending aorta, probably by collateral vessels or formation of new vessels. Morphological changes ranged from broad bands of necrosis to patchy areas of cell loss, primarily in middle layers of descending aorta. We conclude that vasa vasorum are critical in nourishment of aortic media.