Manganese superoxide dismutase (MnSOD), a critical mitochondrial antioxidant enzyme, has been described to be tyrosine nitrated and inactivated in various pathologies associated with overproduction of nitric oxide (•NO). Previously we have shown that under anaerobic conditions MnSOD (E. coli) catalyzed the NO conversion into nitrosonium (NO⁺) and nitroxyl (HNO/NO^-) species. In the present paper we show that under aerobic conditions MnSOD stimulates •NO decay. Upon sustained exposure to •NO MnSOD serves as a catalyst and target of •NO-mediated peroxynitrite (ONOO^-) formation, enzyme tyrosine nitration and oxidation which culminates in MnSOD inactivation. Substantial amounts of H₂O₂, (derived from O₂^•-, an intermediary in ONOO^-formation through a reaction of NO^-with molecular oxygen) were detected during the decay of •NO in the presence of MnSOD. The results of the present study suggest that MnSOD may play a novel dual role in the formation and decomposition/scavenging of ONOO^-. The data also indicate that MnSOD`s capability to serve as a potent catalyst for self nitration can account for molecular specificity of MnSOD nitration in vivo. The interaction of •NO with MnSOD may represent a novel mechanism by which MnSOD protects the cell from deleterious effects associated with overproduction of •NO.