Using a basal diet containing an amino acid mixture simulating the composition of polished rice protein, the effect of amino acid supplementation on growth of rat was studied to explore the difference between the actual effect of supplementation of amino acid to rice diet and the theoretical expectation. When the basal diet contained 6% of amino acid mixture simulating rice protein, single supplementation of 0.4% of L-Lys did not stimulate the growth of rat but the supplementation of L-Lys together with 0.24% of DL-Thr improved greatly the growth just like the case of amino acid supplementation to the natural rice diet. This result may imply the importance of the interrelationships between dietary Lys and Thr in addition to the availability of Thr in rice protein provided the diet containing an amino acid mixture can neglect the problem of digestion of amino acids in food protein. When the level of amino acid mixture in basal diet was increased to 8%, however, significant improvement of growth was observed with single supplementation of Lys. Some problems relating to the basic principles of amino acid supplementation are also discussed.