Methylene blue photosensitized oxidation of tyrosine in the presence of nitrite produces 3-nitrotyrosine, with maximum yield at pH 6. The formation of 3-nitrotyrosine requires oxygen and increases using deuterium oxide as solvent, suggesting the involvement of singlet oxygen in the reaction. The detection of dityrosine as an additional reaction product suggests that the first step in the interaction of tyrosine with singlet oxygen generates tyrosyl radicals which can dimerize to form dityrosine or react with a nitrite-derived species to produce 3-nitrotyrosine. Although the chemical identity of the nitrating species has not been established, the possible generation of nitrogen dioxide (NO2) by indirect oxidation of nitrite by intermediately produced tyrosyl radical, via electron transfer, is proposed. One important implication of the results of this study is that the oxidation of tyrosine by singlet oxygen in the presence of nitrite may represent an alternative or additional pathway of 3-nitrotyrosine formation of potential importance in oxidative injures such as during inflammatory processes.