The influx of [3H]phenylalanine, [3H]tyrosine and [3H]tryptophan into brain cells was studied using brain slices from adult rats. Each aromatic amino acid inhibited the influx of the others into the brain cells. Tryptophan inhibited non-competitively the influx of phenylalanine, and phenylalanine similarly that of tyrosine and tryptophan. On the other hand, tyrosine inhibited competitively the influx of phenylalanine, and similarly tryptophan that of tyrosine, and tyrosine that of tryptophan. Among the aromatic organic acids tested, only phenylpyruvate and homogentisate had any inhibitory effect on the influx of the aromatic amino acids. These effects were generally competitive, non-competitive inhibition being obtained only in the inhibition of phenylalanine influx by homogentisate. The existence of only one common transport system for aromatic amino acids appear to be unlikely.