The present study was aimed to evaluate the effect of the free radical scavenger Edaravone on infarct volume due to permanent MCA occlusion in mice and, if so, to elucidate the mechanism of its neuroprotective effects. Male Balb/c mice were subjected to permanent middle cerebral artery occlusion and were treated with 3.0 mg/kg of Edaravone or vehicle 30 min before ischemia. Infarct volume was assessed by 2,3,5-triphenyltetrazolium chloride (TTC) method after 24 h. Furthermore, in situ detection of superoxide in the ipsilateral neocortex was carried out using the superoxide-sensitive dye dihydroethidium (DHE) staining technique. Pretreatment with 3.0 mg/kg of Edaravone ameliorated the tissue damage in the infarct rim and significantly reduced infarct volume to about 77% of the control (p<0.05). Semi-quantitative measurement of red fluorescence emitted from DHE revealed that the superoxide increased in the ischemic core at 1 h after the onset of ischemia and extended towards the infarct rim at 3 and 6 h, and that pretreatment with 3.0 mg/kg of Edaravone significantly inhibited the increase of superoxide in the infarct rim at 3 and 6 h (p<0.01). Double staining with DHE and monoclonal antibody against NeuN showed that the majority of the nuclei positive for DHE were also positive for NeuN. These findings suggest that Edaravone salvages the boundary zone of infarct by scavenging reactive oxygen species especially in the neurons during permanent focal cerebral ischemia.