Tryptophan hydroxylase (EC 1.14.16.4) from rat mid-brain is inactivated upon exposure to oxygen. The degree of inactivation is dependent both on the temperature and partial pressure of oxygen to which the enzyme is exposed. Furthermore, molecular oxygen, and not an oxygen or hydroxyl radical, is responsible for the inactivation. Sulfhydryl compounds and reductants partially protect the hydroxylase from inactivation by oxygen. Enzyme inhibited by oxygen can be reconstituted by anaerobic incubation in the presence of dithiothreitol and Fe2+ at 25 degrees C and in some experiments the inclusion of inorganic sulfide, in addition to dithiothreitol and Fe2+, led to even greater recoveries of activity. Preincubation of tryptophan hydroxylase with various sulfhydryl reagents or disulfide compounds also produces inactivation which can be rapidly reversed by dithiothreitol. The substrate tryptophan protects the enzyme from inactivation by sulfhydryl reagents and disulfides but not from inactivation by oxygen. Finally, the enzyme was inhibited by a variety of iron chelators. These results suggest that the catalytic activity of tryptophan hydroxylase is dependent on the oxidation-reduction status of--SH groups and iron sites, which are probably located at the catalytic (substrate binding) site of the enzyme.