To estimate the oxygen consumption rate of arteriolar walls we measured the intra- and perivascular partial oxygen tension (PO2) of arterioles in rat cremaster muscle, both at resting and vasodilating states by phosphorescence quenching technique. Using measured intra- and perivascular PO2 values, oxygen consumption rates were determined based on Krogh-Erlang analysis. We found a significant drop in PO2 in the arterioles after branching. We found that the vascular oxygen consumption rates of functional arterioles were 100-1000 times greater than those seen in in vitro experiments. These results strongly support the hypothesis that arteriolar walls consume a significant amount of oxygen as compared with the surrounding tissue. Furthermore, the reduction of vascular tone of arteriolar walls may facilitate an efficient supply of oxygen to the surrounding tissue.