Background Atrioventricular junctional (AV nodal) reentrant tachycardia can be cured by catheter ablation of the slow pathway, which is part of the reentrant circuit.Previous work has suggested that extracellular double potentials may help identify the site of the slow pathway, but the origin and significance of these potentials are controversial. The aim of this study was to identify the source of these potentials.Methods and Results Studies were performed in isolated, blood-perfused porcine (n=8) and canine (n=4) hearts. Several methods were used to identify the origin of potentials: microelectrode recording, extracellular mapping, pacing from multiple sites, and light microscopy. Two types of double potentials, similar to those found in humans, were found in all hearts. LH potentials consisted of a low-frequency deflection followed by a high-frequency deflection during sinus rhythm or anterior septal pacing. HL potentials consisted of a high-frequency deflection followed by a low-frequency deflection. LH potentials were found close to the coronary sinus orifice. They were caused by asynchronous activation of the sinus septum and the region between the coronary sinus orifice and tricuspid annulus. HL double potentials were found along the tricuspid annulus. They were caused by asynchronous activation of two cell layers. The high-frequency component was caused by depolarization of atrial-type cells in the deep subendocardial layer. The low-frequency component was caused by depolarization of cells with nodal characteristics close to the endocardium. These cells were present around the entire tricuspid annulus, were not part of the compact AV node, and could be dissociated from the bulk of the atria by rapid atrial pacing.Conclusions LH potentials are caused by asynchronous activation of muscle bundles above and below the coronary sinus orifice.Their proximity to the site of the slow pathway is probably serendipity. HL double potentials are caused by asynchronous activation of atrial cells and a band of nodal-type cells close to the tricuspid annulus. The band of nodal-type cells is not part of the compact AV node and may represent the substrate of the slow AV nodal pathway. (Circulation. 1994;89:2351-2360.)