Epicardial electrodes were chronically implanted on the surface of atria and ventricles in 60 patients at the time of open-heart surgery. Eighteen patients developed bigeminy or multiple premature ventricular contractions associated with slow heart rates in the immediate postoperative period. The ectopic ventricular activity was abolished by pacing the atria or ventricles faster than the spontaneous cardiac rate. In seven patients, low cardiac output associated with low heart rates was improved by pacing at more optimal rates. When feasible, atrial pacing is preferred to ventricular pacing to utilize atrial systole to improve ventricular filling and increase cardiac output. The safety and ease of implantation of these electrodes enable them to be used routinely for diagnostic and therapeutic application in all patients undergoing open-heart surgery.