OBJECTIVES: We sought to study the role of IKACh in atrial fibrillation (AF) and the potential electrophysiologic effects of a specific IKACh antagonist. BACKGROUND: IKACh mediates much of the cardiac responses to vagal stimulation. Vagal stimulation predisposes to AF, but the specific role of IKACh in the generation of AF and the electrophysiologic effects of specific IKACh blockade have not been studied. METHODS: Adult wild-type (WT) and IKACh-deficient knockout (KO) mice were studied in the absence and presence of the muscarinic receptor agonist carbachol. The electrophysiologic features of KO mice were compared with those of WT mice to assess the potential effects of a specific IKACh antagonist. RESULTS: Atrial fibrillation lasting for a mean of 5.7 ± 11 min was initiated in 10 of 14 WT mice in the presence of carbachol, but not in the absence of carbachol. Atrial arrhythmia could not be induced in KO mice. Ventricular tachyarrhythmia could not be induced in either type of mouse. Sinus node recovery times after carbachol and sinus cycle lengths were shorter and ventricular effective refractory periods were greater in KO mice than in WT mice. There was no significant difference between KO and WT mice in AV node function. CONCLUSIONS: Activation of IKACh predisposed to AF and lack of IKACh prevented AF. It is likely that IKACh plays a crucial role in the generation of AF in mice. Specific IKACh blockers might be useful for the treatment of AF without significant adverse effects on the atrioventricular node or the ventricles.