This study was prospectively planned to realize the reduction of calcium ion in serum along with the cardiopulmonary bypass[CPB], to find out the cause of the reduction, and to verify the justification of the classical methods of calcium replacement. Nine patients with various open heart surgeries by CPB in 1987 wee selected at random. Calcium chloride was added as follows:: For each unit of ACD blood transfusion, 600mg of calcium chloride was added. In case of massive transfusion, 600 mg of calcium chloride was injected every 2 or 3 units of transfusion. On occasions such as weaning from CPB, or following defibrillation, or hypotension, weak myocardial contractility of the heart, calcium chloride was needed in an amount of 10 mg / kg. In ICU, calcium chloride was limited to use in low serum level or in emergency use. Total calcium decreased early bypass and progressively increased above the preoperative value during late bypass and three hours thereafter, Ionized calcium increased during late bypass and three hours following. Total and ionized calcium depicted similar patterns of change during open heart surgery. Decrease of the calcium at the early bypass was thought from reduction of total protein and alkalosis during bypass. Meanwhile, increase of both calciums during the end of surgery was presumably attributable to addition of calcium chloride in priming solution, injections of calcium chloride in the process of termination of bypass. We conclude that enough calcium was replaced by the classical methods of calcium supplement.