A 3-4 cm length of sulcus terminalis tissue including the sinoatrial node (SAN) was excised from 14 dogs. After an initial junctional rhythm with SAN excision a P wave emerged within days to weeks in 12 animals. Maximum heart rates of the SAN (preoperative) in response to exercise (276 +/- 15 beats/min), isoproterenol infusion in conscious animals (272 +/- 11 beats/min), and stellate stimulation during anesthesia (273 +/- 9 beats/min) were significantly greater than subsidiary pacemakers (postoperatively) for exercise (219 +/- 9 beats/min), isoproterenol (226 +/- 8 beats/min), and stellate stimulation (197 +/- 9 beats/min). During a final experiment, electrophysiological mapping of the area of earliest epicardial activation (pacemaker location) was carried out. By use of a suction electrode in reference to plunge electrodes located in the anterior interatrial band (AIB), eustachian ridge of the coronary sinus, and limbus of the fossa ovalis, the pacemaker was located at the inferior vena cava-inferior right atrial junction in 80% of the animals mapped. During isoproterenol infusion the foci shifted to regions of the AIB in 70% of the animals mapped. The concept of pacemaker hierarchy is discussed.