Oxygenation of the arterial wall is provided by diffusion of oxygen outward from the main vessel lumen and inward from the adventitial vasa vasorum. In a group of four Yucatan miniature pigs the oxygenation profiles across the superficial femoral arteries were recorded by polarographic oxygen microelectrodes. The profiles obtained suggested a relatively poorly oxygenated media (a trough value of approximately 25% that of the intimal oxygenation) with a progressive rise in oxygenation toward the intimal and adventitial surfaces. In four other survival experiments, occlusion of the adventitial vasa vasorum by flush ligation of the arterial branches that supply them resulted in the production of a focal, intimal hyperplastic lesion that was absent in control vessels (intimal to medial ratios [mean±SEM] of 0.053±0.008, n=8,p<0.001 and 0.013±0.001,n=8, respectively). By electron microscopy this lesion was seen to be composed mainly of smooth muscle cells. This evidence would support the hypothesis that arterial wall hypoxia may be involved in tllie initiation of intimal hyperplasia. It is proposed that human atherosclerosis may be initiated by occlusion of the vasa vasorum and concomitant hypoxia.