After feeding marmosets different lipid supplements for 6 months, the distribution of phospholipid classes and the fatty acid composition of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and diphosphatidylglycerol (DPG) were determined in their cardiac membranes. Supplementing the diet with linoleic-acid-rich sunflower seed oil raised the level of 18:2, n-6 in both PC and PE, but did not change the level significantly in DPG. When 18:2, n-6 was increased, the level of arachidonic acid (20:4, n-6) was significantly decreased in PC and PE. No arachidonic acid was present in DPG. Supplementing the diet with mutton fat did not markedly increase the level of saturated fats, nor did it markedly reduce the level of arachidonic acid in any phospholipid component. No dietary treatment altered the distribution of the major phospholipid classes.