Changes in plasma level of arginine vasopressin (AVP), arterial pressure, and urine flow were studied before, during and after cardiopulmonary bypass (CPB) in 11 patients with congenital heart disease. Anesthesia was induced with thiopental sodium (3-5 mg/kg) and was maintained with enflurane (1.0-1.5%), 50% N2O in O2 and morphine (0.5 mg/kg). Concentration of plasma AVP increased slightly from 3.8 ± 1.5 pg/ml after induction and increased 3-fold after sternotomy. Plasma AVP level increased to 132 ± 26 pg/ml and 218 ± 54 pg/ml after 5 and 60 min on CPB, respectively. When the circulation returned to normal, plasma AVP level decreased gradually but was still significantly higher at 24 hr (13.4 ± 2.5 pg/ml). Marked osmolar diuresis was induced with mannitol in the priming solution used during the CPB: increases in urine flow, Na excretion and osmolar clearance. Possible mechanisms of marked increase in AVP release and differences of AVP responses during CPB reported by other investigators are discussed.