The aim of the present study is to compare the influence of timing and duration of mild hypothermia on rats subjected to 3 h of middle cerebral artery occlusion followed by 72 h of reperfusion. Sixty-four Sprague-Dawley rats were divided into three mild hypothermic groups according to the duration of mild hypothermia (MHT 32 +/- 0.2 degrees C): intra-ischemia (MHTi); intra-reperfusion (MHTr); and intra-ischemia/ reperfusion (MHTi + r). Our control group was normothermic (NT 37 +/- 0.2 degrees C). Reversible focal cerebral ischemia was carried out in rats with a suture technique. Cerebral blood flow was measured by laser Doppler flowmetry to confirm occlusion and reperfusion. The permeability of the blood-brain barrier was determined by the extravasation of Evans's blue dye, and infarct volumes were measured by 2,3,5-triphenyltetrazolium chloride staining at 72 h after reperfusion. Acute post-ischemic hyperperfusion and delayed hypoperfusion in the ischemic perifocal area and sustained hypoperfusion in the ischemic core were inhibited in MHTi + r and MHTi rats (p < 0.05) as compared to the NT rats. The action of MHTi + r on preventing post-ischemic progressive hypoperfusion in the perifocal area was more effective than that of MHTi 2 h after reperfusion (p < 0.05). Blood-brain barrier disruption in the basal ganglia and cortex areas was significantly reduced in both MHTi + r and MHTi groups, and especially the former. Infarct volume was significantly reduced in both MHTi and MHTi + r groups (p < 0.05). MHTi and MHTi + r have protective effects for reducing ischemia/reperfusion injury. The potential mechanisms may include inhibition of post-ischemic hyperperfusion, and delayed and sustained hypoperfusion.