Three cases are described in which accelerated atrioventricular conduction occurred during an acute myocardial infarction. The first patient, an 82-year-old woman, developed a WPW syndrome suggesting posterior right ventricular preexcitation, a pattern which persisted for four months until her death. An accessory bundle was found on autopsy. Fibrotic changes, associated with acute lesions (hemorrhage, polymorphonuclear infiltrates) were present in the atrioventricular node and His-Purkinje system. Two men, of 47 and 74 years, developed a short PR interval associated with supraventricular tachycardia during the course of an acute myocardial infarction. The PR interval returned to its initial value in one case and remained unchanged for three months in the other. Accessory atrioventricular connections which became functional during myocardial ischemia may explain the various electrocardiographic patterns of preexcitation.