Nitrogen balance was studied in 12 uremic patients given nitrogen intakes in varying formulations, either solely as amino acids or as keto acid and amino acid mixtures. It was shown that of the three formulas studied, the one that was most effective in promoting positive balance was the one having threonine as reference unit, with a relatively high tyrosine intake and lower methionine content, and histidine in the proportions found in human milk and in the egg pattern. Also, in all instances in which an amino acid formulation caused a deviation in nitrogen balance, the same effect was obtained with formulations containing keto acids. However, the effect with the keto acids was less pronounced.