Recent evidence has indicated that functional activation of cerebral cortex is accompanied by increases of blood flow and glucose consumption but not oxygen consumption. No explanation has been advanced for this change of the flow-metabolism couple. We formulated the hypothesis that oxygen delivery to brain tissue is diffusion-limited by the enormous hemoglobin binding, and rate-limiting for the oxygen consumption of the tissue. One prediction of this hypothesis is very low oxygen tensions in the tissue. A second prediction is the inability of oxygen consumption to increase during functional activation in the absence of recruitment of capillaries for the oxygen diffusion capacity. We designed a study to test the latter prediction by calculating the density of functioning capillaries during vibrotactile stimulation of the parietal cortex. We defined functioning capillaries as capillaries that transport glucose and therefore calculated the capillary density from the glucose diffusion capacity (K1) of the cerebral capillaries. We confirmed the presence of a partial flow-CMRglc couple (2:1) during the functional activation. Oxygen consumption did not change despite an increase of capillary density in proportion to the change of blood flow.