The variations of growth gain and the composition of free amino acid levels in plasma and erythrocytes of young rats(wistar strain male) were determined by microbioassay method, feeding diets of rice group and 7% casein group as a control for three weeks. The results were as follows; 1. The growth gain of control diet group was higher than the rice diet group. 2. The contents of free tryptophan, lysine, and threonine levels in plasma and erythrocytes on rats of 7% casein group were higher than the rice group. 3. In the 7% casein diet group and the rice group, these free amino acids were included more in erythrocytes than in plasma. 4. Therefore, generally feeding by high protein score diet was included more Try, Lys, Thr in plasma and erythrocytes than feeding by low protein score diet. So the high and low of protein score was assumed by the contents of Try, Lys, Thr in plasma and erythrocytes on rats.