Background. The pig has become an increasingly popular model for the study of cerebral protection during cardiothoracic surgery in recent years, but little information is available concerning hypothermic porcine physiology. Because the efficacy of cerebral protection depends largely upon metabolic suppression, we studied cerebral oxygen metabolism at various temperatures using two different methods to assess cerebral blood flow (CBF). Material and Methods. Twelve pigs (7 to 13 kg) underwent cooling on cardiopulmonary bypass to 8degreesC as recorded by an electrode placed deep in the parenchyma of the brain. CBF was measured in 6 animals using radioactive microspheres and in the other 6 using fluorescent microspheres. CBF, cerebral oxygen consumption, and cerebral vascular resistance were determined at 37degreesC, 28degreesC, 18degreesC, and 8degreesC. Results. Both methods produced very similar data. CBF fell steadily with decrease in temperature to 18degreesC but failed to drop further with more profound hypothermia. With both groups combined, mean cerebral oxygen metabolism was 2.63 mL/100 g per minute at 37degreesC. Metabolic activity was 50% of base line values at 28degreesC, 19% at 18degreesC, and 11% at 8degreesC. The Q(10) value in the pig-the degree of metabolic suppression achieved by a 10degreesC drop in temperature-is 2.46 (95% confidence interval 2.1 to 2.9); this value is consistent with similar studies in humans. Conclusions. The presence of significant residual metabolic activity at 18degreesC suggests that this degree of hypothermia may provide incomplete cerebral protection during prolonged interruption of CBF. This study demonstrates that cooling to temperatures below 18degreesC in the pig can achieve greater metabolic suppression although it may be associated with loss of cerebral autoregulation.