Experimental and clinical cases have been described in which bradycardia, i.e., heart rates below 60 beats/min or slowing of the heart rate, resulted in lethal ventricular arrhythmias during various stages of myocardial ischemia and infarction. The present study was designed to determine the relationship of lethal ventricular arrhythmias and slow heart rates. In 18 dogs anesthetized with sodium pentobarbital, the left anterior descending (LAD) coronary artery was ligated. Standard ECGs, His bundle electrograms and composite electrograms from intramural and epicardial areas in ischemic and normal zones were recorded during the first 3 hours of ischemia. Vagosympathetic trunk stimulation caused varying degrees of slowing and bradycardia. Of the 18 dogs, slowing of the heart rate or marked bradycardia induced ventricular ectopic beats coupled to the sinus beats in two, sustained ventricular tachycardia in two, and ventricular fibrillation in two. In another group of six dogs studied 17-25 days after LAD ligation, one dog showed sustained ventricular tachycardia in response to vagal-induced bradycardia. In all acute or chronic cases of arrhythmias after LAD ligation, continuous electrical activity was recorded on one or more of the electrograms within or overlying the ischemic or infarcted zones. This bridging electrical activation, which is indicative of slow conduction, provided strong presumptive evidence for reentry as the mechanism of lethal or potentially lethal ventricular arrhythmias triggered by bradycardia in the setting of myocardial infarction.