To study the role of nitric oxide (NO) in regulating oxygen consumption by vessel walls, the oxygen consumption rate of arteriolar walls in rat cremaster muscle was measured during enhancement and inhibition of NO synthesis. The oxygen consumption rate of arteriolar walls was calculated based on the intraand peri-vascular oxygen tension (PO2) measured by phosphorescence quenching microscopy. The inhibition of NO synthesis increased the oxygen consumption rate of the vessel walls by 42%, whereas enhancement of flow-induced NO release decreased it by 34%. These results suggest that NO plays an important role not only as a regulator of peripheral vascular tone, but also as a modulator of tissue oxygen consumption by reducing oxygen consumption by vessel walls.