A 64-year-old male was admitted to hospital because of repeated episodes of syncope and palpitation. Ambulatory monitoring revealed paroxysmal atrial fibrillation (AF) as the cause of palpitation; he did not have structural heart disease. The induction of AF by rapid pacing (50 Hz for 1 s) in an upright position provoked syncope with a vasodepressor response. Atropine sulfate blocked the induction of syncope. The possible etiology was neurally mediated syncope that manifested only during AF, which suggests that the abnormal vagal activity during AF in this case exaggerated the vasodepessor response while upright.