Multidose potassium cardioplegia is a common method of myocardial preservation. Although initial potassium arrest conserves high-energy phosphates, there is conflicting evidence that repeat high potassium boluses augment this protection. Fifty-six patients were prospectively randomized to receive multidose cold high potassium cardioplegia (27 mEq of KCl/L) both in the initial and subsequent infusions (Group 1) or an initial cold high potassium (27 mEq/L) cardioplegia followed by boluses of cold low potassium (7 mEq, of KCl/L) solution (Group 2). The two groups were compared in terms of postoperative myocardial electrical stability and hemodynamic performance. Electrocardiograms were recorded by continuous Holter monitor, and the data were analyzed by computer. The duration of aortic cross-clamping and cardiopulmonary bypass did not differ between groups. Group 1, who received more total KCl than Group 2 (p less than .005), experienced more high-grade ventricular ectopia during both reperfusion (p less than .001) and the immediate postoperative period (p less than .001), and required more lidocaine hydrochloride (p less than .001) for arrhythmias. There was no significant difference in hemodynamic performance between the two groups. This study fails to show an advantage to multidose "high potassium" cardioplegia and found a significant increase in ventricular ectopia associated with its use. We advocate using low potassium solutions after initial cold high potassium arrest.