There are many cAUSES OF ALTERED AMINO ACID AND PROTEIN METABOLISM IN UREMIA WHICH MAY Lead to impaired growth, wasting, malnutrition, and other aspects of the uremic syndrome. These causes have complex interrelationships that are not well understood. The factors include altered nutrition due to poor intake, losses of nutrients during dialysis, and abnormal metabolism of many nutrients. Uremic toxins, superimposed catabolic illnesses, elevated or reduced serum hormone levels, reduced capacity of the kidney to synthesize certain amino acids and to degrade other amino acids, peptides, and small proteins, and decreased excretion of certain amino acids and peptides may also contribute to altered amino acid and protein metabolism. The response of certain plasma amino acids to protein restriction appears to differ in uremic patients as compared to normal subjects. Increased plasma levels of many products of amino acids and proteins in renal failure are due primarily to decreased urinary clearance by the kidney. However, for some metabolites, increased synthesis or decreased degradation may also contribute to elevated levels. These latter compounds include guanidinosuccinic acid, methylguanidine, certain middle molecules, and in some patients, phenylpyruvic acid.