Myocardial ischemia followed by reperfusion was produced in artificially respirated, open-chest rats. Coronary artery ligation for 6 min rarely evoked arrhythmias; however, reperfusion after this period rapidly produced severe dysrhythmias in all control animals. Reperfusion after 12 min of ischemia produced less frequent dysrhythmias than after coronary occlusion for 6 min. Feeding of a linoleic acid-rich diet, applying 12% sunflower seed oil in rat food pellet for 4 weeks, decreased the incidence of reperfusion-induced ventricular fibrillation both after 6 min (2/15 vs. 7/11) and 12 min (0/11 vs. 2/8) of myocardial ischemia and the incidence of other arrhythmias was also decreased. The number of animals developing no arrhythmias during reperfusion was increased (8/15 after 6 min of ischemia, 4/11 after 12 min of ischemia vs. 0/11 and 0/8 in controls, respectively). Our results indicate that increased dietary consumption of linoleic acid decreased the occurrence of life-threatening arrhythmias both during the acute phase of myocardial ischemia and during reperfusion in anesthetized rats.