Arginine deficiency is known to lead to marked alterations in pyrimidine biosynthesis and the excessive loss of urinary orotic acid. Orotic acid feeding is known to lead to hepatic steatosis. These studies show that arginine deficiency also results in a marked increase in liver lipids in the rat. The majority of the increased liver lipid can be accounted for by triglyceride accumulation. Increased liver lipid infiltration was found to be independent of the sex of the rat. Accompanying this increase was a decrease in serum triglycerides and cholesterol concentrations. Fatty infiltrations induced by arginine deficiency could be reversed by refeeding an arginine enriched diet. Adenine supplementation (0.30%) to the arginine deficient diet also completely prevented the induction of fatty livers. Adenine supplementation resulted in a dramatic increase in urinary orotic acid excretion in the arginine deficient rat. Guanine supplementation (0.5%) to an arginine deficient diet reduced but did not prevent the induction of fatty livers. The similarities of fatty livers induced by arginine deficiency and orotic acid feeding are discussed.