Introduction: As an adjunctive method to conventional ablation of atrial fibrillation (AF), ablation of sites showing high-frequency spectral component (HFC) during sinus rhythm, known as AF nest, has been introduced. Locations of some HFC sites are similar with those of ganglionated plexi. However it has not been elucidated whether vagal stimulation can alter the spectrum pattern of atrial electrogram. Methods: Seven anesthetized open-chest dogs were evaluated. Atrial electrograms during sinus rhythm were obtained epicardially from the right atrium (appendage, crista terminalis, free wall, and the sinus node), the left atrium (appendage, free wall, and right and left pulmonary vein antrum), and the coronary sinus. Vagal stimulation was performed to double the sinus cycle length with 20 Hz, 0.2 ms, 4-10 V at both cervical vagus nerves. HFC was defined as an area of frequency > 70 Hz in power spectrum curve of each electrogram. Results: Total 52 sites were analyzed. The sinus node area (4/5 sites), crista terminalis (6/9), inferior portion of left atrial appendage os (3/4), and mid portion of the coronary sinus (3/5) were the major sites showing HFC at basal state and during vagal stimulation. However, there was no statistically significant differences in spectrum pattern and HFC (HFC area / total area: 0.353짹0.178 vs. 0.391짹0.214, p = 0.051) between basal state and during vagal stimulation. Conclusion: HFC of atrial electrogram during sinus rhythm seemed to be mostly resulted from tissue characteristics of the underlying myocardial structure rather than autonomic tone.