We studied the effect of a long-term diet rich in linoleic acid (LAR diet) on the acute phase of experimental myocardial infarction. Male Sprague-Dawley CFY rats were fed a diet containing 12% sunflower seed oil or standard laboratory chow for 3 months. Gas-chromatographic determination of the fatty acid content in total heart phospholipids showed an increase in the n-6/n-3 fatty acid ratio, resulting from an elevated arachidonic acid content of the membrane. In parallel experiments in conscious animals, the left anterior descending coronary artery was ligated by a previously implanted silk loop. Long-term LAR diet increased the survival rate from 19 to 81% and reduced the occurrence of arrhythmias during the first 20 min after coronary ligation. Oral administration of 10 mg/kg indomethacin 1 hr prior to coronary ligation abolished the protective effect of the LAR diet, while 200 mg/kg aspirin did not. The prevention by indomethacin of the beneficial effect suggest the involvement of an increased prostaglandin synthesis in the protection offered by the diet. The ineffectiveness of aspirin in this respect is discussed. These results provide evidence that a long-term LAR diet may be of therapeutic value in the management of myocardial infarction.