How the brain senses local reductions in O2 partial pressure (pO2) that occur following neuronal activity is unknown, but it has now been shown that small decreases in pO2 induce Ca2+ responses in astrocytes. Reductions in O2 availability inhibits astrocytic mitochondrial respiration, leading to a cascade of events that culminates in IP3 receptor activation and the release of ATP, which acts on brainstem respiratory rhythm-generating neurons to increase breathing.