Effects of 1% dietary orotic acid on triglyceride metabolism were examined in SD-rats and Kud: ddY mice. When rats were fed semisynthetic diet containing 1% orotic acid and n-6 polyunsaturated fatty acid (linoleic acid), the hepared diet. In contrast to rats which respond to orotic acid consumption with increases in hepatic triglyceride content, mice did not so respond. The rats-limiting step in triglyceride synthesis is catalyzed by the enzyme phosphatic acid phosphohydrolase (EC3.1.3.4) which is present in the liver cytosol and microsomes of rats fed oroic acid diet. This finding suggests that the activity of this enzyme may play a tole in the fatty liver formation in rats.