Introduction: Catheter ablation targeting complex fractionated atrial electrograms (CFAE) is one of the novel techniques used in atrial fibrillation (AF) management. In addition, ablation of sites showing high-frequency spectral component (HFC) during sinus rhythm, known as AF nest, has been introduced as an adjunctive method to conventional AF ablation. Locations of some sites showing HFC are similar with CFAE sites, but it has not been systematically evaluated whether these two targets indicate the same foci. Methods: Five anesthetized open-chest dogs were evaluated. Atrial electrograms were obtained epicardially. AF was induced by burst atrial pacing with 20 Hz during vagal stimulation. Vagal stimulation was performed to double the sinus cycle length with 20 Hz, 0.2 ms, 4-10 V at both cervical vagus nerves. CFAE sites were determined during AF according to the electrogram patterns. Atrial electrograms during sinus rhythm were also obtained from the CFAE sites (n=10) and non-CFAE sites (n=30). Spectrum pattern and areas of HFC were assessed for AF nest. HFC was defined as an area of frequency > 70 Hz in power spectrum curve of each electrogram. Results: Major CFAE sites were high and mid crista terminalis areas, right and left pulmonary vein antrum, and mid portion of the coronary sinus. Among them, only 60% of CFAE sites showed AF nest during sinus rhythm. Interestingly, a part of non-CFAE sites (47%) also showed AF nest. Conclusion: CFAE sites were not the same with the sites showing HFC in this animal model with vagally-mediated AF. Therefore these two kinds of ablation methods would target the different substrates of AF.