Aim of this study is to investigate, by use of the pulse radiolysis technique, the mechanism of oxidation of trypthophan by OH and secondary inorganic radicals. Parallel pulse radiolysis studies were, performed on tryptophan and 1-formyltryptophan. The results seem to indicate that the most important attack on tryptophan residues is an one-electron oxidation. These results are compared with other data from the literature and a general discussion is made.