Background Complex fractionated atrial electrograms (CFAE) has been a possible target for radiofrequency catheter ablation (RFCA) of atrial fibrillation (AF). However, the mechanism of CFAE has not been clearly elucidated. We designed in vivo acutely-induced AF canine model to evaluate whether CFAE during acutely-induced AF changes under the effect of vagal nerve stimulation (VNS) and the spatial distribution of CFAE in that condition. Methods and Results 5 anesthetized canines were prepared for cervical vagal nerve stimulation (VNS, high frequency 20 Hz) and electrophysiologic study. We induced AF by rapid atrial pacing or atrial extrastimulation. CFAE software of 3-D electroanatomical mapping (NavX) was used to construct a contact CFAE map of the left atrium (LA) during AF. Contact bipolar electrograms of AF were acquired for greater than 6 seconds during sustaining AF. CFAE mapping was twice obtained at baseline AF and during VNS, respectively. LA was divided into 4 regions: anterior wall including left atrial appendage, inferior/posterior wall, septum, and pulmonary veins (PV) ostia including LA roof. Regional distribution of CFAE before and after VNS, and its shifting pattern at each animal were analyzed. Results: (1) The mean numbers of identified maximal CFAE sites after VNS (4.2짹1.3) was greater than those before (3짹1.4, p=0.033). (2) Before VNS, CFAE were predominantly observed in the PV or PV-LA junctions, however they were evenly distributed in all 4 predefined LA regions during VNS. (3) In 2 of 5 canines, maximal CFAEs site of the PVs clearly shifted to the outside of the PVs and 9 newly identified CFAEs with VNS were distributed at anterior wall in 3, at septum in 3, at the inferior wall in 1, and 2 around the PV areas. Conclusion The spatial distribution of CFAE in AF during VNS was predominantly shifted to the outside of PVs, compared to those inside or PV-LA junctions before VNS, and the number of maximal CFAEs was increased by VNS. These findings suggest that vagal contribution to the mechanism of CFAEs in AF was mostly found outside of the PVs, which could be identified by shifting pattern of CFAEs by acute modulation of autonomic nervous system. Key Words: Atrial fibrillation, Complex fractionated atrial electrogram, Autonomic nervous system