The effect of mild hypothermia (MH) on microcirculation after resuscitation from cardiac arrest is controversial. The aim of this study was to determine whether MH improves or aggravates the disturbance of cerebral microcirculation. Twenty domestic male pigs were randomized into the MH group (n = 8), non-hypothermia (NH) group (n = 8) or sham operation group (n = 4). In the MH group, the animals were initiated rapid intravascular cooling at 1 h after return of spontaneous circulation (ROSC) from 8 min ventricular fibrillation, and the core temperature was reduced to 33 A degrees C for 12 h and then rewarmed to 37 A degrees C. In the NH group, animals did not receive hypothermia treatment after ROSC. In the sham operation group, the same surgical procedure was performed, but without inducing ventricular fibrillation and hypothermia treatment. The cerebral microvascular flow index (MFI) of large microvessel (diameter > 20 mu m) and small microvessel (diameter < 20 mu m) was measured after ROSC. Cerebral oxygen extraction ratio, internal jugular venous-artery lactate difference, and CO2 difference were also calculated. Cerebral MFI dramatically reduced after ROSC, and MH further aggravated the decrease in MFI of small microvessel compared with NH (p < 0.05). Internal jugular venous-arterial lactate difference and CO2 difference, and oxygen extraction ratio were all significantly increased after ROSC. MH significantly decreased the values compared with NH (p < 0.05). MH decreases cerebral small microvessel blood flow and cerebral metabolism after ROSC compared with NH. However, the total effect is that cerebral oxygen supply-demand relationship is improved during hypothermia.