THE aim of this study was to investigate whether the neuroprotective properties of magnesium in cerebral ischaemia involve suppression of repetitive tissue depolarizations. Cortical spreading depressions (CSDs), evoked by cortical KCl application, and cardiac arrestinduced anoxic depolarization (AD) were measured by extracellular DC recording on intact rat brain. At 90 min after onset of CSDs saline, MK-801 (3 mg/kg) or MgSO4(90 mg/kg) was given i.v. Latency time to AD was measured after 4h. The frequency of CSDs was significantly reduced in animals treated with MgSO4or MK-801. AD was significantly delayed by MgSO4but not by MK-801. Our results suggest that suppression of depolarization by magnesium may play a role in its neuroprotective properties in cerebral ischaemia.