The effects of unhydrogenated and partially-hydrogenated marine and rapeseed oils on the relative content of n-6 fatty acids in rat liver phospholipids were studied by gas-liquid chromatography. The type of dietary oil strongly affected the pattern of n-6 fatty acids in the liver phospholipids even after a relatively short feeding period (3 weeks). With diets deficient in linoleic acid, accumulation of the 20:3 n-9 fatty acid was observed in the animals receiving partially hydrogenated oils, but not in the animals receiving the unhydrogenated marine oil which contained relatively high amounts of polyunsaturated fatty acids of the n-3 family. The results presented are in line with reports in the literature that dietary trans fatty acids may inhibit the desaturation-elongation enzyme systems of rat liver. The observed effects did not appear to be directly correlated to the dietary content of very-long-chain monounsaturated fatty acids.