We measured blood vessel wall oxygen tension (PO2) in vivo in the tissue and on the surface of dog thoracic aortas and femoral arteries. The overall mean tissue PO<sub>2</sub> in the outer vascular wall supplied by vasa vasorum for both blood vessels was 49.4 ± 2.6 (SE) Torr for 42 sites in 8 dogs. Changes in PO<sub>2</sub> were measured after intravenous injections of adenosine and epinephrine. There was some tendency for the PO<sub>2</sub> to decrease with epinephrine for both vessels. With adenosine the thoracic aorta wall tissue PO2 increased significantly (+23.2 ± 6.0 Torr, p < 0.025), while the femoral artery did not. One possible interpretation of these results is that the vasa vasorum of the thoracic aorta are capable of changing blood flow and oxygen supply to the outer wall to a greater degree than the femoral artery.