The experiment was undertaken to determine the optimal amino acid solution for total parenteral nutrition (TPN) in 20 rats. Four different amino acid solutions were used, namely A, B, C and D, of which the E/N ratios were in order 0.8, 1.0, 2.0 and 3.0. TPN was introduced to rats of non-operated and operated groups, using these amino acids. Body weight change, nitrogen balance and plasma and urine aminograms were studied for assessment of the effect. The results were as follows; Non-operated group: Although all groups showed weight gain and positive nitrogen balance, group B with an E/N ratio of 1.0 was the best, followed by C, A then D. Blood aminograms showed markedly elevated levels of Thr in all groups, especially in B, C and D. This may be due to overdosage of Thr for rats. D group had increased urinary loss of Lys, Thr and Glu. These data suggest that B was the best amino acid solution for TPN in the non-operated group. Operated group: A small intestinal bypass operation was performed in rats and TPN was begun after operation. Weight gain and positive nitrogen balance were less than in the non-operated group. The best result was obtained in group C. Marked rise of Thr was observed as in the non-operated group. The typical pattern of postoperative change of aminogram was not seen, i.e., Phe, Met, branched amino acid did not increase. The aminogram of group C was close to the normal pattern. In the operated group, C with an E/N ratio of 2.0 was the optimal amino acid solution.