It has been shown that hypercholesterolemia and atherosclerosis can be induced in rabbits by feeding a diet devoid of cholesterol. It was also found that some essential amino acids produced hypercholesterolemia and hypertriglyceridemia in rabbits. Plasma levels of essential amino acids were found to be elevated in hyperlipidemic patients. It is not known whether the effects of those essential amino acids can be produced in other species such as rats, or whether the effect may be induced by a single amino acid. Therefore, it was thought of interest to study the effect of chronic treatment with essential amino acids on the plasma levels of cholesterol, triglyceride and total lipids and to correlate these results with their effect on plasma amino acid levels in rats.