The utilization of cardiopulmonary bypass in neonates, infants and children often requires the use of deep hypothermia at 18 degrees C with occasional periods of circulatory arrest. Thus, marked physiologic extremes of temperature and perfusion are induced. The safety of these techniques appears to be related to the reduction of metabolism, particularly cerebral metabolism. We studied the effect of deep hypothermic circulatory arrest and cardiopulmonary bypass on brain oxygenation using near-infrared spectroscopy. After hypothermic arrest, brain oxygen extraction during severe hypoxia and severe hypotension is diminished. However, these responses remain intact after cardiopulmonary bypass. Additionally, cardiopulmonary bypass, rather than deep hypothermic circulatory arrest alters the cerebral oxygen response to hypercapnia. The primary goal of studying alteration of brain oxygenation during cardiopulmonary bypass and deep hypothermic circulatory arrest is to improve our understanding of the association between these methods and perturbations in hemodynamics and ventilation, so that effective brain protection strategies can be developed.