Peroxynitrite is known to be a strong oxidant and a nitrating agent of aromatic phenolic or heterocyclic rings, depending on its form in aqueous solutions: the anion ONOO − , its conjugate acid ONOOH, or its CO 2 adduct ONOOCO 2 − . Various reactions have been observed with melatonin (1), a tryptophan derivative, in phosphate-buffered solutions. Melatonin (1) is recognized as a scavenger of several strong oxidants (HO · , H 2 O 2 , ...) accounting for its biological and pharmacological effects. Here we describe two oxidation routes that give rise to indol-2-ones 2 (probably via a 2,3-epoxyindole) and kynuramines 6 (by cleavage of the pyrrole ring), attributable to reactions of ONOOH and ONOO − , respectively, according to the effects of pH and CO 2 content. At pH = 7.6 and in the presence of CO 2 , an important conversion is the cyclization of the lateral amide function, giving 3-substituted pyrroloindoles 4. At neutral pH, therefore, all routes coexist, with a balance between indol-2-ones 2 and pyrroloindoles 4 on the one side and kynuramines 6 on the other, depending on the