Surgical disruption of the small (approximately 0.7 x 1.0 cm) epicardial fat pad situated at the junction of the inferior vena cava (IVC) and inferior surface of the left atrium (ILA) interrupts both right and left vagal input to the atrioventricular nodal (AVN) region of the canine heart. This intervention eliminates AV block during supramaximal stimulation of both cervical vagi, without interfering with sinus bradycardia normally associated with sinoatrial nodal (SAN) suppression. Independent modulation of SAN and AVN activities by the parasympathetic system is thereby revealed. Histology of the excised IVC-ILA fat pad reveals multiple well organized autonomic ganglia. These ganglia range from 2 to 80 cells per cluster and are associated with numerous nerve trunks. Individual ganglia are commonly surrounded by fatty connective tissue closely adjacent to epicardial muscle. They have not been found imbedded within atrial muscle and have been been found in or close to endocardial muscle layers. Other ganglia, imbedded in the fat pad overlying the posterior surface of the left atrium or in the atrioventricular groove, do not directly modulate A-V conduction. Surgical dissection around the extreme left or middle segments of the great cardiac vein and the coronary sinus failed to interrupt either left or right vagal input to the AVN region. Parasympathetic, preganglionic pathways to these AVN synapses do not, therefore, course from left to right along the atrioventricular groove. However, dissection around the extreme right portion of the coronary sinus at its penetration of the inferior interatrial septum, did interrupt vagal influences upon A-V conduction. Thus, numerous autonomic ganglia have been localized in the canine heart which serve as synaptic stations mediating both right and left vagal regulation of A-V conduction.