Objectives: Existing evidence suggests that fractionated atrial electrograms might be associated with underlying ganglionic activity, and that ablation of sites with high spectral frequency might be beneficial for AF treatment. However, the relationship between disorganized AF electrograms and increased vagal tone has not been elucidated. This study was designed to demonstrate the effects of vagal stimulation (VS) on AF disorganization. Methods: 7 mongrel dogs were studied. Epicardial bipolar electrograms were recorded from the free wall of both RA and LA. VS was applied at the right cervical vagus. AF was induced by burst pacing at baseline and during VS. AF electrograms were digitally filtered, and a FFT was performed on a 10-s window of each AF episode to determine the dominance frequency (DF) and to calculate the organization index (OI) as the ratio of the area of the first four harmonic peaks to the total area of the amplitude spectrum. Results: VS increased dominant frequency of AF electrograms in both RA and LA (*p<0.05). However, at baseline there was LA-to-RA frequency gradient while in contrast, during right cervical VS, RA had higher DF. The OI was significantly reduced by VS in both atria (*p<0.05). Importantly, during VS the organizational disparity between the atria was accentuated and RA became more disorganized than LA (#p<0.05). Conclusion: VS increased DF of during AF, altered the DF gradient, and significantly reduced the OI. These changes might underlie the mechanistic role of the vagus in AF initiation and maintenance.