The livers of the wistar strain male rats which had been mentioned in our study were analyzed by the micro-bioassay method for comparison of the levels of free branch-amino acids(valine, leucine, isoleucine) and phenylalanine, methionine, tyrosine. Rats which were fed on diet of higher protein score contained correspondingly higher level of free branch-amino acid in their liver while the levels of phenylalanine, methionine, tyrosine showed unrecognizable relationship with the protein score of the diet used.