To study the effect of deep hypothermic cardiopulmonary bypass and total circulatory arrest on cerebral metabolism and oxygenation, we measured the cerebral metabolic rate for oxygen (CMRO2) and assessed brain oxygenation by near infrared spectroscopy before, during, and after hypothermic bypass in 15 pediatric patients. One group underwent repair during deep hypothermic bypass (18 degrees C) with continuous flow (n = 9); the second group underwent deep hypothermic bypass with total circulatory arrest (n = 6). In the continuous-flow group, CMRO2 returned to control during rewarming and after cardiopulmonary bypass, as did oxyhemoglobin and deoxyhemoglobin in brain tissue. In the total circulatory arrest group, the oxyhemoglobin and the oxidation state of cytochrome aa3 oxidase decreased significantly during circulatory arrest. After cardiopulmonary bypass, the cytochrome oxidation state and the CMRO2 were significantly lower than control measurements, and brain tissue deoxyhemoglobin was elevated. Results of this study indicate that intracellular brain oxygenation decreases significantly during circulatory arrest and remains impaired after rewarming and cardiopulmonary bypass despite normalization of oxygen availability.