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One of the most important functions of the blood circulation is [O.sub.2] delivery to the tissue. This process occurs primarily in microvessels that also regulate blood flow and are the site of many metabolic processes that require [O.sub.2]. We measured the intraluminal and perivascular p[O.sub.2] in rat mesenteric arterioles in vivo by using noninvasive phosphorescence quenching microscopy. From these measurements, we calculated the rate at which [O.sub.2] diffuses out of microvessels from the blood. The rate of [O.sub.2] efflux and the [O.sub.2] gradients found in the immediate vicinity of arterioles indicate the presence of a large [O.sub.2] sink at the interface between blood and tissue, a region that includes smooth muscle and endothelium. Mass balance analyses show that the loss of [O.sub.2] from the arterioles in this vascular bed primarily is caused by [O.sub.2] consumption in the microvascular wall. The high metabolic rate of the vessel wall relative to parenchymal tissue in the rat mesentery suggests that in addition to serving as a conduit for the delivery of [O.sub.2] the microvasculature has other functions that require a significant amount of [O.sub.2].