Plasma and urinary concentrations of different amino acids were investigated during diabetic ketoacidosis (DKA) and 12, 24, 72 hours after initiation of therapy. In DKA, plasma concentration of glutamic acid, aspartic acid, valine, leucine and isoleucine significantly increased while that of asparagine and glutamine decreased compared to levels in well-controlled diabetic patients. The urinary excretion of branched-chain amino acids, histidine, serine and threonine was elevated while those of glutamic acid, glutamine, glycine and taurine were reduced. Among the different amino acids, histidine excretion had the highest variability. A strong correlation was found between the urinary excretion of several amino acids and that of the beta-2-microglobulin characterizing tubular dysfunction. Changes in the excretion of different amino acids reflect the altered metabolic state and renal function due to DKA.