Backgrounds; Vagal nerve stimulation (VNS) has been known to promote atrial fibrillatioin (AF) by decreasing atrial effective refractory period (AERP) and increasing the spatial heterogeneity of refractoriness. However, the mechanisms by which AF sustains under the VNS remain to be determined. Methods; In 5 open-chest canine models, AERP and AF inducibility were assessed at 7 atrial sites at baseline and during cervical VNS (5V, 20ms, 20Hz). Using non-contact mapping system (ESI 3000), propagating wavefronts by potential maps and dominant frequency (DF) of virtual ECGs using Fast Fourier Transform (FFT) were analyzed during AF on/off VNS. After vagal denervation (VD) by 2% lidocaine block at the third fat pad, AF inducibility and wavefront dynamics were reassessed during VNS. Results; VNS shortened AERP at all sites (from 118.4짹17.0 to 56.0짹9.7 ms, p<0.001) and increased the covariance of AERP in left atrium (LA, from 6.5짹5.3 to 21.4짹13.4 %, p=0.049), but not in right atrium (RA, from 8.4짹4.7 to 9.8짹4.6 %, p=0.653). AF could be easily induced by premature extrastimuli and maintained as long as VNS was continued. AF was immediately terminated after cessation of VNS. During AF on VNS, organized reentered wavefronts using similar propagating routs were repetitively identified at the area between LA appendage and left pulmonary veins. The cycle length (CL) of these repetitive activations well correlated with DF of their respective ECGs (CL, 103.4짹8.6 ms; DF, 10.3짹1.2 Hz). In contrast, randomly propagating wavefronts with shorter CL (49.7짹13.2 ms) were observed in RA. When VNS turned off, CL of AF was significantly increased from 103.4짹8.6 ms to 113.5짹10.3 ms in LA (p=0.039) and from 49.7짹13.2 ms to 109.4짹9.1 ms in RA (p=0.002) and changed to atrial flutter-like activity on virtual ECGs. VD (n=2) effectively prevented from induction of sustained AF during VNS. Conclusions; Vagally mediated AF is not entirely random process and can be maintained through relatively stable a single or small number of reentrant wavefronts. This pattern is more obvious in LA where dispersion of refractoriness primarily existed. Effect of VD is mostly exerted through halting this driving mechanism of AF.