Recent experiments have revealed synapses that selectively mediate right and left vagal regulation of sinoatrial function in the fat pad overlying and surrounding the right pulmonary vein complex. However, precise vagal postganglionic pathways to the sinoatrial region have remained obscure. Such pathways, including critically important neural inputs to sinoatrial and atrioventricular nodal regions, may be vulnerable to surgical approaches to atrial or intracardiac repair. The present experiments seek to delineate specific autonomic pathways to the sinoatrial region of the canine heart. The distal ends of the cut right and left cervical vagi and the right and left ansae subclaviae were electrically stimulated (10 to 20 Hz, 1 msec, 2 to 3 V) before and after surgical incisions were placed. Cut No. 1 was made longitudinally along the ventral caval surface from the pericardial reflection caudally to the pulmonary vein fat pad, cut No. 2 was made from the caudal end of cut No. 1 transversely across the sulcus terminalis to a point midway across the anterior (ventral) surface of the right atrium. Each incision was closed with 4-0 silk, with care being taken to avoid injury to either the sinoatrial nodal or the pulmonary fat pad regions. In four of seven animals, these two incisions totally interrupted vagal input to the sinoatrial node, whereas in the remaining three dogs a residual inhibitory influence remained. These residual fibers were surgically ablated by excision of globular fat pads situated on the rostrodorsal surfaces of the right superior pulmonary vein, suggesting a dorsorostral route into the interatrial septum and thence to the sinoatrial node. There was little or no interruption of either right or left vagal input to the atrioventricular nodal region; sympathetic supplies to both sinoatrial nodal and atrioventricular nodal regions remained essentially intact after the two incisions. Thus the major parasympathetic postganglionic projections to the sinoatrial node in the dog heart are by way of the free wall of the right atrium and are vulnerable to surgical interventions in this portion of the heart.