In 41 patients undergoing cardiac operations with extracorporeal circulation, oxidized cytochrome a,a3(CtO2), deoxygenated hemoglobin (Hb), and oxygenated hemoglobin (HbO2) were measured in brain tissue by near-infrared spectrophotometry (NIRS) intraoperatively. Monitoring also included electroencephalography (EEG) and jugular-bulb venous saturation (SBJO2). All operations were performed using membrane oxygenators, moderate hypothermia (26-28 degrees C) and pH alpha-stat management. During cardiopulmonary bypass (CPB) CtO2 and HbO2) were reduced, reaching minimal values when rewarming was instituted. At the end of the operation CtO2 and HbO2 had regained initial levels. During CPB, arterial PCO2, pH, and temperature were closely related to CtO2 (r = 1000, r = -0.964 and 0.929 respectively; p < 0.001, p < 0.001, and p < 0.003 respectively). Neuropsychological testing by the Mini-Mental-State Test indicated reversible postoperative neuropsychological deficits in four patients. There patients had a lower CtO2 minimum compared to those without these deficits (-4.5 mumol/L v. -0.7 mumol/L; p = 0.036). These findings support the hypothesis that neuropsychological deficits in patients after cardiac surgery can be caused by intraoperative cerebral hypoxia.