Modification of tyrosine residues and formation of 3-nitrotyrosine is one of the most commonly identified effects of reactive nitrogen species on proteins. In this study we evaluated the presence and localization of tyrosine nitration in various ventilatory and limb muscles. We also assessed the contribution of the neuronal (nNOS), the endothelial (eNOS), and the inducible (iNOS) isoforms of nitric oxide synthase (NOS) to tyrosine nitration in skeletal muscles both under normal conditions and in response to severe sepsis. In normal rats and mice, muscle tyrosine nitration was detected at 52, 48, 40, 30, 18, and 10 kD protein bands. Tyrosine nitration of the majority of these protein bands was significantly reduced within 1 h of in vivo NOS inhibition in rats. Diaphragmatic protein tyrosine nitration in mice deficient in the inducible NOS (iNOS-/-) averaged ~ 50% of that detected in wild-type (iNOS+/+) mice. Injection of bacterial lipopolysaccharides (LPS) in rats produced a significant rise in protein tyrosine nitration in the mitochondrial and membrane fractions but not in the cytosol of ventilatory muscles. Absence of iNOS expression (iNOS-/-), but not nNOS (nNOS-/-) or eNOS (eNOS-/-), in genetically altered mice resulted in a significant reduction in LPS-mediated rise in diaphragmatic nitrotyrosine. We conclude that tyrosine nitration of proteins occurs in normal muscle fibers and is dependent mainly on the activity of the iNOS isoform. Sepsis-mediated increase in protein tyrosine nitration is limited to the mitochondria and cell membrane and is highly dependent on the activity of the iNOS but not the nNOS or eNOS isoforms.