Pial arterioles and arteries, normally responsive to alterations in PaCO2, were obtained by microdissection from the parietal cortex of 53 anesthetized cats. The oxygen consumption of these vessels, determined with the Cartesian diver microrespirometer, was found to be size dependent. It increased progressively from vessels 60 mum in diameter to vessels 250 mum in diameter. In vessels 250--375 mum in diameter there was a progressive decrease in oxygen consumption with increasing vessel size. The oxygen consumption was not altered by increasing the oxygen concentration in the diver from 20% to 95%, or that of glucose from 1.1 to 2.0 mM. Planimetry of histological sections showed that the percentage of endothelium and smooth muscle increased as vessel diameter increased from 60 to 200 mum, and thereafter decreased with increasing vessel size. When these differences in the composition of the vessel wall were taken into account, there was still residual dependence of the oxygen consumption on vessel diameter. This may represent intrinsic differences in the metabolic rate of vascular smooth muscle in vessels of different size.