Abstract We studied 15 anesthetized dogs with 4-day-old anterior wall infarctions caused by previous ligation of the left anterior descending coronary artery. Left circumflex artery (LCA) stenosis induced an average 41% decrease in collateral-dependent flow to the surviving anterior wall (epicardium) and only a 10% decrease to the normal posterior epicardial wall. Electrogram changes in 12 dogs consisted of fractionation and delay of anterior wall electrograms leading to bridging electrical activity between sinus beats and ventricular ectopic beats in five dogs. No similar electrogram changes were noted in posterior wall recordings. Ventricular paced beats induced ventricular tachycardia arising from the anterior wall in four dogs only after LCA stenosis. LCA stenosis in anterior myocardial infarction may be arrhythmogenic because of increased sensitivity of the surviving tissues, which have been electrically deranged by the ischemic/infarction process.