Rats were fed diets containing either ethyl oleate or ethyl elaidate with and without supplementation of ethyl linoleate. Feeding elaidate increased the ratio of the phospholipids phosphatidylethanolamine to phosphatidylcholine, and inhibited the formation of higher polyunsaturated fatty acids from linoleic as well as from oleic and palmitoleic acids. Elaidic acid was incorporated mainly in the phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol and phosphatidylserine fractions and the level found was linoleate-independent. The 5-cis, 9-transoctadecadienoic acid was deposited only in the phosphatidylinositol and phosphatidylserine fraction when linoleate was fed, but also in phosphaditylethanolamine and phosphatidylcholine with a linoleate deficiency. In contrast to the trans monoene being predominantly in the α1 opposition of phosphatidylethanolamine and phosphatidylcholine, the cis, trans-diene was all in the β position of the phospholipid