Patients with intracerebral hemorrhage (ICH) may deteriorate progressively after the initial ictus because of the brain edema around the hematoma. Recently, thrombin has become known to play an important role in the brain edema formation after ICH. In this study, we examined the effect of brain hypothermia on brain edema formation after hematoma and thrombin injection into the brain in rats and clarified the mechanism of hypothermia on brain damage. Anesthetized Sprague-Dawley rats received an injection of 100 microL of autologous blood or 10 units of bovine thrombin into the basal ganglia. Animals were divided into the normothermic and hypothermic groups, which were housed in a room at 25 degrees C and in a cold room at 5 degrees C respectively, for 24 hours. Brain water content was significantly reduced with hypothermia in the cortex (80.8 vs. 79.7% p < 0.05) after hematoma induction. After thrombin injection, brain water content was also significantly reduced with hypothermia in the basal ganglia (84.5 vs. 82.2%; p < 0.01), accompanied by a significant reduction in blood-brain barrier (BBB) permeability to Evan's blue (29.4 vs. 11.6 ng/g tissue; p < 0.05) and in accumulation of polymorphonuclear leukocytes (3.03 vs. 0.27 U of myeloperoxidase/g tissue; p < 0.01). This study indicates that brain hypothermia significantly reduces brain edema formation after hematoma and thrombin injection into the brain in rats. Inhibition of thrombin-induced BBB breakdown and inflammatory response with hypothermia appear to contribute to brain protection in this model.