The posterior atrial fat pad (PAFP) has been described as the probable anatomic location of parasympathetic ganglia mediating sinoatrial (SAN) and atrioventricular nodal function in the mammalian heart. This contrasts with recent localizations of such control elements in the pulmonary vein fat pad (PVFP) and in fatty tissues overlying the junction of inferior vena cava-inferior left atrium (IVC-ILA), respectively. Short bursts (5-8 pulses/burst, 3 bursts/train) of electrical current (1-16 Hz, 400 ms, 1-5 mA) applied directly to the ventral right atrial epicardium via a concentric bipolar electrode (separation 0.3-0.6 mm) during the atrial muscle refractory period, activated subepicardial postganglionic pathways from PVFP and entering the SAN; identical stimulation of dorsal right atrial epicardium between PAFP and SAN excited few or no fiber pathways controlling SAN discharge rate or patterns. In a second series of experiments, injection of a neuronal marker (Fast Blue) into and around SAN, with time (5-10 days) allowed for retrograde transport, resulted in staining of many soma in PVFP but none in IVC-ILA or PAFP. These data strongly affirm the primary, and perhaps exclusive, localization of ganglia that mediate parasympathetic regulation of SAN function in PVFP of the dog's heart, with little or no such participation by ganglia within PAFP or IVC-ILA.