Endotoxins (lipopolysaccharides; LPS) are known to cause multiple organ failure, including renal dysfunction. LPS triggers the synthesis and release of cytokines and the vasodilator nitric oxide (NO.). A major contributor to the increase in NO(.)production is LPS-stimulated expression of inducible nitric oxide synthase (iNOS). This occurs in vasculature and most organs including the kidney. During endotoxemia, NO. and superoxide react spontaneously to form the potent and versatile oxidant peroxynitrite (ONOO- stop) and the formation of 3-nitrotyrosine (nTyr)-protein adducts is a reliable biomarker of ONOO- generation. Therefore, the present study was aimed at investigating the role of endogenous nitric oxide in regulating Na+,K+-ATPase activity in the kidney, and at investigating the possible contribution of reactive nitrogen species (RNS) by measuring of iNOS activity. In addition, the present study was aimed at investigating the relationship between nTyr formation with iNOS and Na+,K+-ATPase activities. Previously in our study, nTyr was not detectable in kidney of normal control animals but was detected markedly in LPS exposed animals. In this study, kidney Na+, K+-ATPase activity were maximally inhibited 6 h after LPS injection (P:0.000) and LPS treatment significantly increased iNOS activity of kidney (P:0.000). The regression analysis revealed a very close correlation between Na+, K+-ATPase activity and nTyr levels of LPS treated animals (r = -0.868, P = 0.001). Na+, K+-ATPase activity were also negatively correlated with iNOS activity (r = -0.877, P = 0.001) in inflamed kidney. These data suggest that NO. and ONOO. contribute to the development of oxidant injury. Furthermore, the source of NO. may be iNOS. iNOS are expressed by the kidney, and their activity may increase following LPS administration. In addition, NO. and ONOO- formation inhibited Na+, K+-ATPase activity. This results also have strongly suggested that bacterial LPS disturbs activity of membrane Na+, K+-ATPase that may be an important component leading to the pathological consequences such as renal dysfunction in which the production of RNS are increased as in the case of LPS challenge.