The effect of dietary fish oil on myocardial phospholipids and ischemic damage to the heart was studied in the rat. Four weeks of feeding 5% (i.e., 12 energy percent) menhaden oil (MO) produced both profound changes in the fatty acyl composition of phospholipids in myocardial membranes and a significant reduction in the loss of creatine kinase following coronary artery ligation compared with feeding 5% (i.e., 12 energy percent) corn oil (CO). The MO diet did not change the content of either phospholipids or cholesterol in the heart. However, dietary MO resulted in significant elevations in the percent of fatty acids in the total phospholipids that were saturated, the n-3/n-6 ratio and the double-bond index. The changes in total phospholipids were not uniform for all phospholipid classes. Although the n-3/n-6 ratio was increased in each of the individual phospholipids examined, the predominant n-3 fatty acid incorporated (i.e., 20:5, 22:5, 22:6) differed among the major phospholipid classes. Also, the percent saturation was elevated in phosphatidylcholine with no change in double-bond index, whereas both the percent saturation and double-bond index were elevated in phosphatidylethanolamine. Thus dietary MO resulted in selective alterations in individual myocardial phospholipids. These membrane changes may be involved in the observed reduction of ischemic damage in the heart.