Amino acids were determined in plasma and urine from controls and children with chronic renal insufficiency. Significant differences were found in the mean plasma concentration of serine, threonine, glutamic acid, tyrosine, arginine, cystine and glycine in children with severe renal failure. There was a marked increase in clearance of the majority of amino acids when glomerular filtration rate (GFR) fell below 30 ml/min per 1.73 m2: the exceptions were the basic amino acids and cystine which did not change. As GFR decreased there was a significant reduction in the percentage tubular reabsorption of individual amino acids. Children with severe renal failure excreted up to 40% of the filtered load of amino acids in the urine. This significant increase in urinary amino acid loss may contribute to the derangement of plasma amino acid concentrations found in uremia.