Biochemical aspects of total parenteral alimentation in 10 patients (two low birth weight newborn infants and eight infants) for periods of 5 to 24 days were studied by: A. Determination of concentration of amino acids and ammonia of two commercially available protein hydrolysates. B. Analyses of blood and urine obtained before, during, and after parenteral alimentation for various biochemical parameters including amino acids, urea nitrogen, glucose, and osmolality. The results showed that the protein hydrolysates contained a very high concentration of ammonia. This inordinate amount of ammonia taxes the Krebs urea cycle and in premature infants enhances acidosis and respiratory distress syndrome. It also explains the abnormal liver function tests and hyperammonemia found concomitantly with infusion of protein hydrolysate. Another effect of infusion of protein hydrolysate, in conventional amount, is persistent hyperaminoacidemia.