Plasma amino acid concentrations were serially estimated after 82.5% resection of the liver in dogs. Slight to moderate decreases were found in valine, leucine, isoleucine, threonine and methionine concentrations for the first 12 hours. The first three branched chain amino acids thereafter increased up to 180%. Between 24 and 72 hours, threonine and methionine increased markedly in four surviving dogs but not in two which died in hepatic coma. Significant increases were found in phenylalanine and lysine concentrations but there was no substantial change in tryptophan levels throughout the experiment. Among the non essential amino acids, the most striking increase was seen in tyrosine and serine concentrations with their peak at 48 to 72 hours. Arginine and aspartate transiently decreased within 12 hours but most of other amino acids increased more or less during the observation. Although the molar ratios of the branched chain to the aromatic amino acids dropped dramatically, it was difficult to draw definitive remarks on the relationship between the molar ratio and the grade of hepatic coma in each animal. A further study is necessary to determine whether the correction of plasma amino acid alterations is beneficial or not following critical liver resection.