Plasma amino acids were found to deviate extensively from normal in patients with end-stage uremia. Concentrations of all essential amino acids except phenylalanine and methionine were reduced. Progressive impairment of reabsorption of amino acids by the kidney is found as glomerular filtration rate (GFR) falls, renal clearances increasing fivefold to fortyfold at GFR less than 10 ml/min. Impairment of absorption of amino acids by the small intestine may explain the abnormal plasma curves after oral loading with tryptophan. These and other abnormalities may be the basis for development of abnormal protein metabolism in chronic, progressive renal failure in man.