Astroctyes are thought to regulate cerebral vasoconstriction, but how astrocytes sense haemodynamic stimuli is not known. A new study found that, in rats and mice, astrocytes associated with parenchymal arterioles were activated by fluctuations in blood flow and pressure, and astrocyte activation was required for the vasoconstriction response. The calcium-permeable non-selective cation channel TRPV4, which can be activated by physical stimuli, was required for astrocyte activation in response to haemodynamic stimuli. These findings indicate that TRPV4 is a sensor for haemodynamic stimuli in parenchymal arterioles and helps to regulate arteriole tone and blood flow.