Earlier studies in man could demonstrate that intravenous lipids decreased the concentration of amino acids in plasma (to 50%). In short- and long-term experiments in rat the effect of orally applied olive-oil on the concentration of amino acids in blood, on the excretion of 3-methylhistidine and of total nitrogen into urine was measured. Two control groups were used: rats with total restriction of food and rats which were fed ad libitum. 2 ml olive-oil given to rats within 12 hours decreased the concentration of alanine, valine, leucine, isoleucine. Whereas food restriction increased the excretion of 3-methylhistidine into urine and the blood concentration of leucine, isoleucine, valine, tyrosine, phenylalanine, threonine, and glutamine, the oil-fed rats showed no increase of amino acid levels in plasma and no increase of 3-methylhistidine excretion into urine. These experiments suggest that lipid application inhibits proteolysis in muscle, and thus diminishes the amino acid levels in blood.