In vitro studies demonstrated that peroxynitrite inactivates both human recombinant MnSOD (hrMnSOD) and E. coli MnSOD causing enzyme tyrosine residue(s) nitration. This led to a suggestion that human MnSOD nitration and inactivation in vivo, detected in various deseases associated with oxidative stress and overproduction of nitric monoxide (NO)–conditions that favor peroxynitrite formation–are also caused by peroxynitrite. In a previous study we demonstrated that the exposure of E. coli MnSOD to NO under the anaerobic conditions causes NO conversion (dismutation) into reactive nitrosonium (NO⁺) and nitroxyl (HNO/NO^-) species, which produce enzyme modifications and inactivation (Niketic et al., Free Rad. Biol. Med. 27: 992 (1999)). The present study shows that interaction of NO with E. coli MnSOD leads to the formation of nitrating species capable of nitrating and oxidizing enzyme tyrosine residues, as well as that these species are less invasive than peroxynitrite in producing enzyme modifications and inactivation. Low molecular mass thiols are shown to reduce enzyme inactivation and NO-induced tyrosine nitration. The present study contributes to the understan-ding of the nature of NO reaction with E. coli MnSOD and provides compelling argument in support of the direct involvement of NO in MnSOD mediated generation of nitrating species. Interaction of NO with MnSOD may represent a novel mechanism by which MnSOD protects the cell from deleterious effects associated with overproduction of NO. However, extensive MnSOD modifications and inactivation associated with a prolonged exposure to NO will amplify toxic effects caused by elevated cell superoxide and NO levels.