Dietary orotic acid is known to induce the fatty liver. Fatty acid profiles in the lipid fraction of the liver and the serum in rats fed with or with orotic acid diet were analyzed. In all the hepatic lipid fraction of rats fed on the supplemented orotic acid diet, there was a significant increased in linoleic acid. In addition, linoleic acid was also increased in the triacylglycerol fraction of hepatic endoplasmic reticulum and the triacylglycerol and diacylglycerol fractions of hepatic Golgi apparatus of the orotic acid-feeding rats. In the time course study of the fatty acid profile in the hepatic triacylglycerol and diacylgycerol fractions, an increase of linoleic acid was observed similarly in the initial stages of orotic acid intake in the both fractions. However, linoleic acid in the serum triacylglycerol fraction of orotic acid-feeding rats increased from day 1, but it began to decrease the increment from day 2, resulting in the lower level of linoleic acid in the serum triacylglycerol fraction of orotic acid-feeding rats than that of rat fed a orotic acid-free diet after 10 days. Oleic acid (18:1) was increased in the only cholesteryl ester fraction of helpatic. However, oleic acid level in other fractions was not changed. The compositions of 14:0, 16:0 and 18:0 was reduced in the hepatic triacylogylcerol, diacylglycerol and cholesteryl ester fractions by orotic acid-feeding. However, these saturated fatty acids were significantly increased in the serum triacylglycerol fractions. The orotic acid indcued changes in linoleic acid level in hepatic triacylglycerol may be explained by the impaired fatty acid metabolism and limited excretion of this fatty acid from liver to serum.