The levels of the secreted, interstitially located extracellular superoxide dismutase (EC-SOD), the cytosolic copper-and-zinc–containing SOD (CuZn-SOD), and the mitochondrial manganese-containing SOD (Mn-SOD) were measured in the walls of human coronary arteries, proximal thoracic aortas, and saphenous veins. The blood vessel walls, particularly the arteries, were found to contain exceptionally large amounts of EC-SOD, whereas the levels of CuZn-SOD and Mn-SOD were relatively low compared with other tissues. Analysis of EC-SOD by immunohistochemistry indicates an even distribution in the vessel wall, including large amounts in the arterial intima. Arterial smooth muscle cells were found to secrete large amounts of EC-SOD and likely are the principal source of the enzyme in the vascular wall. The EC-SOD concentration in the human arterial wall extracellular space is high enough to efficiently suppress the putative pathological effects of the superoxide radical, such as oxidation of LDL and reaction with nitric oxide to form the deleterious peroxynitrite. The levels of EC-SOD in the aortic wall are found to vary widely among species and were on average 6440 U/g in humans, 4340 U/g in the cow, 2660 U/g in the pig, 160 U/g in the dog, 770 U/g in the cat, 2390 U/g in the rabbit, 90 U/g in the rat, and 3400 U/g in the mouse. There were only moderate differences in the amounts of CuZn-SOD and Mn-SOD. This wide variation in EC-SOD content suggests that the susceptibility to pathologies induced by superoxide radicals in the vascular wall interstitium should vary widely among species.