P wave morphology in leads II and III, recorded at high gain and sweep speed, was correlated to the initial site of activation over the anatomical SA node and at sites more distal on the sulcus terminalis. Pacemaker localization was inferred from the initial site of activation of seventeen bipolar recording sites on the sulcus terminalis, interatrial septum and left atrium in anesthetized, open chest dogs. Positive P waves were observed with activation originating from the rostral to the caudal sulcus terminalis. However, different initial sites of activation produced P waves with distinct morphological characteristics. Pacemaker localization nearest the rostral SA node produced upright bifid P waves which exhibited a small initial positive deflection forming a plateau on the ascending limb. Recovery to baseline was generally smooth. A middle SA nodal pacemaker produced P waves similar to that of a rostral pacemaker, but the amplitude was higher and a notch appeared halfway during recovery to baseline. The distinguishing features of the P wave with activation originating at the caudal SA node were a bifid initial deflection and multiple notching at the peak of the P wave and during recovery to baseline. Activation originating at the upper and middle third of the sulcus terminalis distal to the anatomical SA node produced P waves whose initial deflection was negative. The peaks of these P waves were notched. Pacemaker localization in the vicinity of the lower sulcus opposite the coronary sinus produced P waves in leads II and III which were very low amplitude appearing as oscillations in the baseline. Thus, a positive P wave occurs in lead II and III not only during sinus rhythm, i.e., activation originating from the vicinity of the anatomical SA node, but also from non-SA nodal sulcus sites. Furthermore, the morphology of the P wave may be used to infer pacemaker localization within the SA node and at other sites along the sulcus terminalis in the supine open or closed chest dog.