The effects of the size of acute myocardial infarction on ventricular reentry and automaticity were studied in 36 mongrel dogs. Large transmural myocardial infarctions were produced by ligation of the left anterior descending coronary artery (major ligation; diameter of infarction above 4.0 cm), and small subendocardial or intramural infarctions were produced by ligating a small diagonal branch of the left anterior descending coronary artery (minor ligation; diameter of infarction less than 1.5 cm). Reentrant arrhythmias were induced by rapid ventricular stimulation, and ventricular automaticity was determined during vagal stimulation. Ventricular automaticity became enhanced only 30 to 45 minutes after both major and minor coronary arterial ligations; however, the animals with major ligations attained a higher level of increased automaticity. While automaticity became enhanced in both groups, reentrant arrhythmias could never be produced artificially (or observed spontaneously) in the animals with small myocardial infarctions. The dependence of the so-called reentrant arrhythmias on the size of the infarction is a major support for the theory of reentry as the basis for these arrhythmias.