BACKGROUND This study analyzed the electrophysiologic properties of interatrial transseptal electrical conduction at Bachmann's bundle and the ostium of the coronary sinus (CS os) in response to pulmonary vein (PV) stimuli, which mimicked spontaneous ectopy. RESULTS Forty patients with atrial fibrillation (AF) referred for ablation (15 persistent AF and 25 paroxysmal AF) and 15 control patients were enrolled in the study. During decremental extra-stimulus pacing from the PVs, right atrial activation was analyzed using noncontact mapping and multipolar catheters. The refractory periods and conduction times were calculated for Bachmann's bundle, CS os, and left atrium. The dispersion of refractoriness was calculated as the difference between the refractory periods of Bachmann's bundle and the CS os. The refractory period at Bachmann's bundle was 244 msec in the persistent AF group, 213 msec in the paroxysmal AF group, and 199 msec for controls. The refractory period at the CS os was 220 msec in the persistent AF group, 201 msec in the paroxysmal AF group, and 193 msec for controls. The dispersion of refractoriness was 54 msec in the persistent AF group; this was significantly greater than in paroxysmal AF at 32 msec (P < 0.05) and controls at 13 msec (P < 0.01). During decremental pacing, lengthening of conduction times at both Bachmann's bundle and the CS os were significantly greater in the persistent AF group compared with paroxysmal AF or control groups. A higher dispersion of refractoriness was associated with a higher incidence of inducible AF and a lower rate of ablation success. CONCLUSIONS There are differences between the left to right interatrial electrical connections between patients with persistent AF, paroxysmal AF and controls.