Magnesium exhibits a range of neuronal and vascular actions that may ameliorate ischaemic CNS insults, including stroke. Significant neuroprotection with magnesium has been observed in different models of focal cerebral ischaemia in many laboratories, with infarct volume reductions between 25 and 61%. Maximal neuroprotection is evident at readily attainable serum concentrations, and neuroprotection is still seen when administration is delayed up to 6 hours after onset of ischaemia. Clinical use of magnesium in pre-eclampsia and acute myocardial infarction confirms its safety and tolerability. Five small trials in acute stroke have reported reduced odds of death or dependence with administration of magnesium, but confidence intervals are wide, and definitive data from ongoing large trials are awaited.