Coronary sinus (CS) pacing has been shown to prevent induction of atrial fibrillation (AF) by suppression of the propensity of atrial premature beats at high right atrium (HRA) to induce local conduction delay at the posterior triangle of Koch. However, other mechanisms of CS pacing in preventing induction of AF have not been explored. This study investigated whether a differential conduction delay exists between the HRA and distal CS pacing in patients with paroxysmal AF but not in patients without AF. Nine patients with atrioventricular reentrant tachycardia utilizing a left accessory pathway undergoing catheter ablation were included in this study. Group 1 consisted of 5 patients with clinically documented paroxysmal AF and group 2 4 patients without a history of AF. The effective refractory periods (ERPs) of HRA, distal CS, and four different left atrial sites were determined. The interatrial conduction time and conduction delay between the HRA and distal CS during HRA or distal CS pacing were measured. The interatrial conduction delay (ICD) from the HRA to the distal CS during HRA pacing was significantly longer than that from the distal CS to the HRA during distal CS pacing in patients of group 1. However, the ICD from the HRA to the distal CS during HRA pacing was not significantly longer than that from the distal CS to the HRA during distal CS pacing in group 2 patients. A differential conduction delay between the HRA and the distal CS pacing is present in this specific population of patients with paroxysmal AF but not in patients without AF. The shorter conduction delay during DCS pacing may contribute to the prevention of induction of AF.