At pH 7.0 in non-"deaired" potassium phosphate buffer, the reactions of L-tyrosine, L-dopa, dopamine, L-norepinephrine, and L-epinephrine with peroxynitrite (PN) or nitrite, generated colored products. These products displayed not only unique colors and patterns of mobility on silica thin layer chromatographic plates, but also varied increase of absorbance between 400 and 540 nm. In particular, with the treatment of nitrite, catecholamines exhibited longer bands and multiple colored spots due to the formation of multiple compounds. In addition, significantly increased mobilities were noted with nitrate-incubated catecholamines. These results imply the occurrence of various types of reactions, such as nitration and nitrosation, via the production of active intermediates of oxygen and/or nitrogen species during incubation.