In this study, we investigated the optimal ratio of individual branched-chain amino acids (BCAA) in a balanced amino acid infusion in laparotomized rats. The total BCAA contents of four amino acid infusions were fixed at 31% of total amino acids. The weight ratios of individual BCAA (isoleucine:leucine:valine) in the solutions were 1:0.5:1, 1:1:1, 1:2:1, and 1:4:1, respectively. The laparotomized rats were infused with about 140 mg (experiment 1) and 100 mg (experiment 2) of nitrogen and 10 g of glucose daily for 7 days. In both experiments, no marked difference was observed in the mean cumulative 7-day nitrogen balance and the urinary 3-methyl-histidine levels of all the groups. The BCAA concentrations and the molar ratios of individual BCAA in plasma were disarranged by the infusion of the 1:0.5:1 and 1:4:1 solutions. The infusion of the 1:1:1 and 1:2:1 solutions tended, however, to allow the values to approach the preinfusion values. These results suggest that the optimal ratio of individual BCAA in an amino acid infusion lies between 1:1:1 and 1:2:1 for this injured rat model in total parenteral nutrition.