Glutathionyl radical (GS • ) formed during the oxidation of glutathione by tetramethylammonium peroxynitrite ([NMe 4 ][ONOO]) was spin-trapped with 5,5′-dimethyl-1-pyrrolineN-oxide (DMPO) and 5-diethoxyphosphoryl-5-methyl-1-pyrrolineN-oxide (DEPMPO). This radical reacted with ammonium formate to form the carbon dioxide anion radical (CO •− 2 ). The superoxide anion formed during oxidation of GSH by peroxynitrite salt was trapped with DMPO and detected as the DMPO-hydroxyl adduct. Addition of SOD mimic completely abolished the spectrum of the hydroxyl adduct but not the spectrum of the DMPO-glutathionyl radical adduct. Addition of seleno-dl-cystine or its reduced form caused a dramatic inhibition in the formation of spin adducts, suggesting that seleno-dl-cysteine is a more effective scavenger of peroxynitrite. The oxygen uptake observed during oxidation of GSH by peroxynitrite salt was inhibited by spin traps. In the presence of catalase, approximately 50% of the oxygen consumed was restored, indicating stoichiometric conversion of O 2 to H 2 O 2 during oxidation of GSH by peroxynitrite salt. Results indicate that nitrite and glutathione disulfide are formed as the major products during oxidation of GSH by peroxynitrite.