The cellular mechanisms underlying functional hyperemia—the coupling of neuronal activation to cerebral blood vessel responses—are not yet known. Here we show in rat cortical slices that the dilation of arterioles triggered by neuronal activity is dependent on glutamate-mediated [Ca²⁺]_i oscillations in astrocytes. Inhibition of these Ca²⁺ responses resulted in the impairment of activity-dependent vasodilation, whereas selective activation—by patch pipette—of single astrocytes that were in contact with arterioles triggered vessel relaxation. We also found that a cyclooxygenase product is centrally involved in this astrocyte-mediated control of arterioles. In vivo blockade of glutamate-mediated [Ca²⁺]_i elevations in astrocytes reduced the blood flow increase in the somatosensory cortex during contralateral forepaw stimulation. Taken together, our findings show that neuron-to-astrocyte signaling is a key mechanism in functional hyperemia.