The present study addresses the relationship between blood flow and glucose consumption in rat primary somatosensory cortex (SI) in vivo. We examined bilateral neuronal and hemodynamic changes and 2-deoxyglucose (2DG) uptake, as measured by autoradiography, in response to unilateral forepaw stimulation. In contrast to the contralateral forepaw area, where neuronal activity, blood oxygenation/flow and 2DG uptake increased in unison, we observed, in the ipsilateral SI, a blood oxygenation/flow decrease and arteriolar vasoconstriction in the presence of increased 2DG uptake. Laminar electrophysiological recordings revealed an increase in ipsilateral spiking consistent with the observed increase in 2DG uptake. The vasoconstriction and the decrease in blood flow in the presence of an increase in 2DG uptake in the ipsilateral SI contradict the prominent metabolic hypothesis regarding the regulation of cerebral blood flow, which postulates that the state of neuroglial energy consumption determines the regional blood flow through the production of vasoactive metabolites. We propose that other factors, such as neuronal (and glial) release of messenger molecules, might play a dominant role in the regulation of blood flow in vivo in response to a physiological stimulus.