Fat-supplemented diets enriched with linoleic acid by the addition of 12% w/w sunflower seed oil or proportionally reduced in linoleic acid by addition of 12% mutton fat were fed to rats for 18 months before the fatty acid composition of perirenal storage fat and myocardial membranes (phospholipids) was determined. Although the fatty acid composition of perirenal fat generally reflected that of the diet, there was an inverse relationship between the consumption of n-6 and the deposition of n-9 fatty acids. In addition, enhanced deposition of oleic acid (18:l, n-9) appears to be related to the dietary intake of stearic acid (18:0). In contrast, in myocardial membranes the n-3 polyunsaturated fatty acids are found to be increased when the intake of n-6 polyunsaturated fatty acids is reduced. This is particularly evident for docosahexaenoic acid (22:6, n-3) which is significantly increased in phosphatidylcholine, phosphatidylethanolamine, and diphosphatidylgycerol fractions of myocardial membranes, when the mutton fat diet was fed. After feeding the sunflower seed oil diet, the increased consumption of linoleic acid produced only small changes in the 18:2, n-6 content of cardiac phosphatidylcholine and phosphatidylethanolamine. These major classes of membrane phospholipids also showed only small increases in 20:4, n-6. In diphosphatidylglycerol, increased 18:2, n-6 also followed increased dietary intake, but this was not accompanied by increased 20:4, n-6. These changes in myocardial phospholipid fatty acid composition are similar to those observed after short-term feeding reported previously and confirm that changes in dietary n-6/n-3 fatty acid intake affect the fatty acid composition of both myocardial membranes and storage fat. These changes persist for the duration of the feeding period.