Serum electrolyte measurements were performed on 30 patients during and after open heart surgery using a vinyl-sheet, bubble disposable oxygenator and 20% hemodilution perfusion. Postoperative hypopotassemia was severe, but the patients recovered in four hours following operation. The hypopotassemia was more marked in cases in which the cardiopulmonary bypass was used only for a short time. Even patients with a negative potassium balance recovered from the hypopotassemia. The cause of it, therefore, is postulated to be a K+ transfer from the extracellular to the intracellular space. In the recovery process the intracellular K+ reappeared in the extracellular space. During the postperfusion hypopotassemia period, the electrocardiographic recordings showed marked ST depressions, but the occurrence of arrhythmias was rare.