To examine the large drop of partial oxygen pressure (P02) in arterioles, the 02 consumption rates of arteriolar walls were determined under physiological conditions. A phosphorescence quenching technique was used to quantify the intra and perivascular P02 values in rat cremaster arterioles. Using the measured P02 values, and a theoretical model, the 02 consumption rates of the arteriolar walls were then estimated. We found that the 02 consumption rate of arterioles was 100 times greater than that seen in in vitro experiments, and the 02 consumption rate under normal conditions was significantly higher than that during vasodilation. Furthermore, the 02 consumption rate was the highest in the upstream arterioles. These findings suggest that the high 02 consumption rates of arteriolar walls depend on the workload of the smooth muscle.