Wistar strain male rats were divided into four different diet groups and the control group was fed on 7-percent casein diet (C), the second with a mixture of 80% of rice and 20% of buckwheat (RW), the third with rice only (R), and the fourth with a mixture of rice and buckwheat of equal nitrogen level (RWS). Each diet group consisted of 6 rats was fed for three weeks by the ad libitum feeding method. The results of the experiment, i.e., the growth gain and change in the level of free amino acids in the plasma and liver determined by the micro-bioassay method were as follows. 1. The group C showed the largest growth gain and the rest did in the order of RW group, R group. and RWS group. 2. Rats fed on diet of high protein score showed high level of free amino acids tryptophan, lysine, and threonine in the plasma and liver except for the case of lysine in the liver. 3. Protein score would be estimated by the level of free amino acids in the liver and plasma.