The kinetics of the incorporation into protein of [3H]phenylalanine, [3H]tyrosine and [3H]tryptophan were studied with homogenates prepared from whole brain of 1-, 7-, 21- and 60-day-old rats. The maximal velocities (Vmax)of incorporation of phenylalanine and tyrosine decreased and the apparent Michaelis-constants (Km) for all three amino acids increased with increasing age of the rats. Tyrosine had the smallest and tryptophan the largest Km values in all age groups. Phenylalanine competitively inhibited the incorporation of tyrosine, but tyrosine inhibited non-competitively the incorporation of phenylalanine. Tryptophan inhibited competitively the incorporation of phenylalanine, but at least partially non-competitively the incorporation of tyrosine. Phenylalanine and tyrosine did not significantly affect the incorporation of tryptophan in homogenates from 60-day-old rats. In 1-day-old rats only a very large excess of phenylalanine or tyrosine inhibited detectably. The Ki for phenylalanine in the incorporation of tyrosine was significantly smaller in 1- than in 60-day-old rats. In every case the inhibition presumably occurred at a single rate-limiting step in the complicated process of incorporation of amino acids into protein.