BACKGROUND: Clinical trials of hypothermic therapy in asphyxiated infants have started recently. However, clinical studies have been delayed by the difficulty in selecting infants with a bad neurological prognosis and by the concern regarding adverse effects of hypothermia. The purpose of this study is to examine the effects of systemic cooling on cerebral metabolism (CMR) and the regional cerebral blood flow (rCBF) in newborn piglets. METHODS: The rCBF in the seven parts of the brain were measured with colored microspheres. The blood samples for the measurement of cerebral oxygen consumption (CMRO2) and cerebral glucose consumption (CMRglc) was collected from the umbilical artery and the superior sagittal sinus. RESULTS: Reductions of cerebral cortex temperature to 32 degrees C decreased blood flow in all brain regions. In particular, blood flow in the brainstem decreased more significantly than in any other region. The total cerebral blood flow (CBF), CMRO2 and CMRglc, respectively, decreased to 32.3+/-3.9 mL/100 g per min, 2.8+/-1.0 mLO2/100 g per min and 22+/-12 mmol/100 g per min at 32 degrees C (41, 53 and 46% of the initial value). The CBF decreased in parallel with CMRO2 and CMRglc down to 35 degrees C, but CBF decreased to a greater extent than CMRO2 and CMRglc at below 35 degrees C. CONCLUSIONS: The indication of hypothermic therapy and the degree of cooling have to be performed very carefully. Systemic cooling is especially dangerous for the total asphyxiated infants who might have damage to the brainstem because the blood flow in the brainstem has significantly decreased during hypothermia.