Abstract: AIM: To study the results of electrophysiological mapping and radiofrequency ablation in atrial fibrillation (AF) mediated by different concentrations of methacholine (Mach) in dogs. METHODS: Six mongrel dogs were chosen. AF was induced and electrophysiological mapping was performed during AF after respective infusion of low, medium and high concentrations of Mach. During AF mediated by low concentration of Mach, one ablation line from the superior to the inferior vena cava in the posterolateral right atrium was first performed in three dogs, followed by linear lesions in the right anterior right atrium. In the other three dogs, only linear lesions in the right anterior right atrium were ablated. During AF mediated by the medium concentration of Mach, Bachmanns bundle was ablated. During AF mediated by the high concentration of Mach, the area showing stable and regular atrial activation patterns was chosen for ablation on the basis of electrophysiological mapping. RESULTS: Atrial activation was more disorganized and AF cycle length was shorter in the trabeculated right atrium than those in the atrial septum and the left atrium during infusion of (1.04±0.37) μg/(kg·min) Mach. Linear ablation in the anterolateral right atrium prevented reinduction of AF, but it became ineffective after increasing the concentration of Mach. Atrial activation was more disorganized and the AF cycle length was shorter in the atrial septum and the left atrium than those in the trabeculated right atrium during infusion of (2.70±0.49) μg/(kg·min) Mach. Ablation in Bachmanns bundle terminated AF in five dogs and prevented reinduction in four dogs, but it also became ineffective after increasing the concentration of Mach. Stable and regular atrial activation existed in the left side of Bachmanns bundle and the base of the left atrial appendage in two dogs during infusion of (5.42±0.97) μg/(kg·min) Mach. Ablation was performed only in one dog, which terminated AF but failed to prevent reinduction. CONCLUSION: The trabeculated right atrium is a critical site for the initiation of AF mediated by the low concentration of Mach. The atrial septum or the left atrium is a critical site for the initiation and maintenance of AF mediated by the medium concentration of Mach. The focal source exists during AF by the high concentration of Mach. The effective areas by ablation for AF are different with different concentrations of Mach.