Cardiopulmonary bypass with moderate hypothermia and hemodilution is an extremely non- physiologic state. The stress responses to cardiopulmonary bypass were evaluated in open heart surgery by measuring the change of plasma cortisol concentration. Patients were anesthetized with fentanyl 20 ug/kg, 50% nitrous oxide in oxygen and a supplementary dose of droperidol and halothane. In each case, at the onset of cardiopulmonary bypass, the hematocrit value was reduced to 25-30% and was maintained around this level during perfusion. Moderate hypothermia was induced to 28-30 degrees C (28.9±1.5 degrees C). Blood samples were obtained after sternotomy, at the onset of cardiopulmonary bypass, during moderate hypothermia and after cardiopulmonary bypass. Plasma cortisol concentration was measured by radioimmunoassay technique. Compared with the pre-cardiopulmonary bypass cortisol value (11.12±2.19 ug/dl), plasma cortisol concentration at the onset of cardiopulmonary bypass (17.39±5.15 ug/dl) was significantly increased (p<0,05). And during cardiopulmonary bypass, especially during the hypothermia period, the plasma cortisol concentration (19.17±8.09 ug/dl) was significantly increased (p<0.05). After cardiopulmonary bypass, the plasma cortisol concentration (43.0±11.2 ug/dl) was significantly increased when compared with other values (p<0.05). We conclueded that the significant increase of plasma cortisol concentration is secondary to the stress response to cardiopulmonry bypass.