Introduction: The mechanism responsible for the generation of ectopic beats in pulmonary veins (PVs) has not been well defined. The present study examines the electrophysiological characteristics of the PVs and other regions of the canine left atrium (LA) under low dose (300 關M) caffeine condition. Methods : Transmembrane action potentials were recorded from left superior PVs, PV-LA junctions (PLJ, atrium < 5 mm from the PV ostium), LA appendage (LAA) or Bachmann���s bundle (BB) in arterially perfused canine LA appendage-PV preparations using floating glass microelectrodes. Rapid atrial pacing (cycle length 140-300 ms, 10 sec) was used to induce delayed afterdepolarizations (DAD) at baseline and under low dose (300 關M) caffeine condition. Results: Spontaneous diastolic depolarization or triggered activity (TA) was not observed in any of the recording area under baseline condition. DAD and TA were induced by caffeine in 4/8 PVs and in 3/8 PLJs, but not in LAA(0/6) or BB(0/5). These TA and DAD were also observed after termination of pacing-induced atrial tachyarrhythmia. DAD was abolished by pretreatment of the atria with verapamil or propranolol 1.0 關M. Conclusion: Spontaneous diastolic depolarization was not present in perfused canine left atria or proximal PV. Pulmonary veins and adjacent areas displayed an increased susceptibility to develop DAD–induced TA under low dose caffeine condition. These distinctive electrophysiologic properties of the PV and PLJ may contribute to the arrhythmogenic substrate responsible for the ectopic activity that initiates atrial fibrillation. Key Words: Pulmonary veins, Calcium, Atrial fibrillation