This clinical study analyzes the effect of potassium cardioplegic solution containing verapamil hydrochloride (1 mg/L) on cardiac conduction after release of the aortic cross-clamp and throughout recovery. Fifty consecutive patients undergoing open-heart operation were studied as a unit for postoperative conduction abnormalities. They were also analyzed in groups based on spontaneous ventricular conversion to regular rhythm (54%) and the need for single DC cardioversion (32%), or multiple DC cardioversions (14%). Results showed that spontaneous ventricular conversion had no relationship to aortic cross-clamp time and that DC cardioversion using 10 Ws had no detrimental effects on the myocardium or incidence of conduction abnormalities. The need for transient intraoperative pacing was lowest with spontaneous ventricular conversion, but not statistically different from single or multiple DC cardioversions. Only 3 patients (6%) required pacing in the intensive care unit. The incidence of postoperative atrial and ventricular arrhythmias was similar in all groups, and no deaths or episodes of malignant ventricular arrhythmias occurred. This study concludes that verapamil potassium cardioplegia is associated with excellent myocardial protection and a high incidence of transient intraoperative dysfunction of the atrioventricular node (70%) but a low incidence of postoperative pacing. Benign postoperative arrhythmias occur, but at hospital discharge, few conduction abnormalities (10%) persist.