The plasma concentration of nineteen amino acids was examined in patients with diabetic ketoacidosis prior to institution of treatment and in healthy controls. A two to three-fold increase in plasma leucine, isoleucine, valine and α-amino butyrate was observed in the diabetics. In contrast a 25 to 40 per cent reduction was noted for the key glycogenic amino acids alanine, glycine, threonine and serine. It is concluded that in the absence of adequate insulin, increased peripheral mobilization of ketogenic amino acids and enhanced hepatic extraction of glycogenic amino acids contribute to the development of the acute diabetic syndrome.