Two amino acid solutions were compared at different levels of nitrogen (N) intake for their effectiveness in promoting N retention in normal and in traumatized rats when given as part of a complete diet under total intravenous feeding conditions. An essential amino acid pattern based on that found in whole egg and one with an essential amino acid pattern based on an egg-potatoe mixture, were equally efficacious in promoting N retention in normal, 300-g rats infused with 75 mg of N and 80 kcal/day. Traumatized rats were in positive N balance on infusions of 400 mg of N/day from either the egg or the egg-protein amino acid solution for the first 5 days postinjury. On day 5 the injured animals showed no significant changes in plasma total protein, albumin, glucose, urea N, total bilirubin, and L-aspartate aminotransferase when compared with each other or with orally fed control rats. The two amino acid solutions were satisfactory sources of amino acids when infused as part of a complete intravenous diet for the purpose of studies in the parenterally fed rat. The results of these studies supported the adequacy of the amino acid requirement data for the adult rat of Nasset (J. Nutr. 61, 555-569, 1957) and of Smith and Johnson (Brit. J. Nutr. 21, 17-27, 1967) but suggested that the NRC values calculated from the data of Benditt et all. (J. Nutr. 40, 335-350, 1950) were too high for several amino acids. An improvement in the surgical cannulation technique is also discussed.