Rates of cerebral blood flow (CBF) and glucose consumption (CMR(glc)) rise in cerebral cortex during continuous stimulation, while the oxygen-glucose index (OGI) declines as an index of mismatched coupling of oxygen consumption (cerebral metabolic rate of oxygen-CMRO(2)) to CBF and CMR(glc). To test whether the mismatch reflects a specific role of aerobic glycolysis during functional brain activation, we determined CBF and CMRO(2) with positron emission tomography (PET) when 12 healthy volunteers executed finger-to-thumb apposition of the right hand. Movements began 1, 10, or 20¿minutes before administration of the radiotracers. In primary and supplementary motor cortices and cerebellum, CBF had increased at 1¿minute of exercise and remained elevated for the duration of the 20-minute session. In contrast, the CMRO(2) numerically had increased insignificantly in left M1 and supplementary motor area at 1¿minute, but had declined significantly at 10¿minutes, returning to baseline at 20¿minutes. As measures of CMR(glc) are impossible during short-term activations, we used measurements of CBF as indices of CMR(glc). The decline of CMRO(2) at 10¿minutes paralleled a calculated decrease of OGI at this time. The implied generation of lactate in the tissue suggested an important hypothetical role of the metabolite as regulator of CBF during activation.