Myocardial ischemia was produced in the left ventricle of the canine heart by a Harris two-stage occlusion of the left anterior descending coronary artery. The lipid content in the ischemic myocardium was analyzed and compared with the control tissue. No significant change in total phospholipid and cholesterol was detected. A 2-fold elevation in the levels of the major lysophospoholipids was observed during acute ventricular arrhythmias at 24 hr after the onset of ischemia. Such increases were not caused by preferential hydrolysis of phospholipid plasmalogens from the parent phospholipids.