With the growing chick brains take out at 9 different times during the period immediately after hatching to 40 days later the author studied the transaminase activity in these brains. The results are follows. 1. In α-amino acids group aspartic acid, alanine, isoleucine, lencine, valine, norvaline. and tyrosine show the transaminase activity as the continuatoin from embryo and phenylalanine commences its transaminase activity. 2. In ω-amino acid group γ-aminobutylic acid, β-alanine, β-oxy-γ-aminobutylic acid, in the diamino acid group ornithine show the transaminase activity, while cysteic acid also as the continuation from the embryo shows the activity in every period examined. 3. Those amino acids showing the transaminase activity have individual peaks during the period from 4 to 10 days after hatching, and thereafter these amino acids take two different courses; namely, on that slightly falls folowing the respective Peak; and one that stays unchanged.