Wistar Strain male rats were divided into four different diet groups and the control group was fed on seven 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 six rats was fed for three weeks by the adlibitum feeding method. The results of the experiment, i, e, the growth gain and change in the level of branch-amino acids in the plasma 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. It was shown that rats fed on diet of higher protein score tend to have higher level of free branch-amino acids in the plasma. 3. Thus, the protein score may be estimated based on the level of the free branch-amino acids in the plasma.