In pediatric literature there are reported good clinical results after administration of nearly all hitherto tested amino acid solutions despite their different composition. In a first series of experiments on 10 premature infants with a commercial amino acid solution, which was developed for adults, there occurred--despite good tolerance and positive nitrogen balance--distinct disturbances in methionine, phenylalanine, tyrosine, proline, glycine, serine and alanine homeostasis. These disturbances nearly reached the extent of inborn errors of amino acid metabolism. It, therefore, seemed necessary to develop an optimal amino acid solution for parenteral nutrition of premature and newborn infants. There followed a second series of experiments on 5 infants with a correspondingly changed amino acid pattern, a third and a fourth one. By an experimental examination, which was carried out in 80 orally fed infants, normal serum values were determined. On the basis of the findings obtained in the course of these examinations and under consideration of well-known requirement rates determined in oral nutrition it was possible to develop an amino acid solution with highly reduced metabolic risk. At the same time the physiological regulation range of some amino acids could be clarified satisfactorily. By this the significance of tyrosine, proline and possibly glycine as essential amino acids for the young infancy was proved. Further examinations on lysine and leucine metabolism are being carried out.