Cerebral blood flow (CBF) is coupled to neuronal activity and is imaged in vivo to map brain activation. CBF is also modified by afferent projection fibres that release vasoactive neurotransmitters in the perivascular region, principally on the astrocyte endfeet that outline cerebral blood vessels. However, the role of astrocytes in the regulation of cerebrovascular tone remains uncertain. Here we determine the impact of intracellular Ca(2+) concentrations (Ca(2+)) in astrocytes on the diameter of small arterioles by using two-photon Ca(2+) uncaging to increase Ca(2+). Vascular constrictions occurred when Ca(2+) waves evoked by uncaging propagated into the astrocyte endfeet and caused large increases in Ca(2+). The vasoactive neurotransmitter noradrenaline increased Ca(2+) in the astrocyte endfeet, the peak of which preceded the onset of arteriole constriction. Depressing increases in astrocyte Ca(2+) with BAPTA inhibited the vascular constrictions in noradrenaline. We find that constrictions induced in the cerebrovasculature by increased Ca(2+) in astrocyte endfeet are generated through the phospholipase A(2)-arachidonic acid pathway and 20-hydroxyeicosatetraenoic acid production. Vasoconstriction by astrocytes is a previously unknown mechanism for the regulation of CBF.