Atherosclerosis is associated with aging based on numerous epidemiologic studies, whereas arterial wall hypoxia has been associated with other risk factors for atherosclerosis. We studied the effect of age on the transarterial wall oxygen gradient in the rat using an oxygen microelectrode. A decrease in oxygen tension in the outer 35% of the artery wall was noted in 36- to 40-week-old rats when compared to 3- to 4-week-old rats. These findings were noted despite no difference in arterial blood oxygen tension between the two groups and prior to any histologic evidence of atherosclerotic lesion formation. Our observations suggest that aging decreases the delivery of oxygen to the outer artery wall.