Using transient focal and global cerebral ischemia models in the rat, we have previously shown that MgSO4 is not neuroprotective unless it is combined with mild hypothermia. This study establishes a therapeutic time window for combined MgSO4 and mild hypothermia treatment after permanent middle cerebral artery occlusion (MCAO). Rats were subjected to permanent intraluminal thread MCAO and animals were treated 2, 4 or 6 h after ischemia with a MgSO4 infusion (360 μmol/kg, then 120 μmol/kg/h) and mild hypothermia (35 °C) or with vehicle for 24 h. At the 2 h time point, treatment with hypothermia alone and MgSO4 alone were also assessed. Infarct volumes were measured 48 h after MCAO induction. After permanent MCAO, combined MgSO4 and hypothermia treatment reduced infarct volumes by 54% at 2 h (P = 0.048) and by 39% at 4 h (P = 0.012), but there was no treatment effect detected at 6 h or in the hypothermia alone or MgSO4 alone groups. These findings support our earlier work highlighting the neuroprotective effect of MgSO4 when combined with mild hypothermia, even when treatment is delayed by several hours.