Effects of dopamine and its precursor amino acids on the activity of tryptophan hydroxylase were examined. They inhibited the enzyme activity prepared from mastocytoma cells in terms of the biopterin cofactor and the substrate l-tryptophan. In relation to the biopterin, tryptophan hydroxylase was found to have two different kinetics, and dopamine inhibited the activity in a non-competitive way to both the components. Dopamine had the highest affinity to the enzyme, followed by l-DOPA and l-tyrosine, while d-tyrosine did not inhibit the activity. In terms of l-tryptophan, l-tyrosine, l-DOPA and dopamine inhibited the enzyme non-competitively and their affinity to the enzyme was in this order. These results indicate that the indoleamine metabolism may be regulated by catecholamines and their related amino acids in the brain.