The myelin sheaths wrapped around axons by oligodendrocytes are crucial for brain function. In ischaemia myelin is damaged in a Ca2+-dependent manner, abolishing action potential propagation(1,2). This has been attributed to glutamate release activating Ca2+-permeable N-methyl-d-aspartate (NMDA) receptors(2-4). Surprisingly, we now show that NMDA does not raise the intracellular Ca2+ concentration ([Ca2+](i)) in mature oligodendrocytes and that, although ischaemia evokes a glutamate-triggered membrane current(4), this is generated by a rise of extracellular [K+] and decrease of membrane K+ conductance. Nevertheless, ischaemia raises oligodendrocyte [Ca2+](i), [Mg2+](i) and [H+](i), and buffering intracellular pH reduces the [Ca2+](i) and [Mg2+](i) increases, showing that these are evoked by the rise of [H+](i). The H+-gated [Ca2+](i) elevation is mediated by channels with characteristics of TRPA1, being inhibited by ruthenium red, isopentenyl pyrophosphate, HC-030031, A967079 or TRPA1 knockout. TRPA1 block reduces myelin damage in ischaemia. These data suggest that TRPA1-containing ion channels could be a therapeutic target in white matter ischaemia.