N-methyl-D-aspartate (NMDA) receptors serve prominent roles in vast physio-pathological conditions including hyperalgesia (defined as augmented pain intensity in response to painful stimuli) associated with central sensitization. Using M40403 a synthetic low molecular weight superoxide dismutase mimetic that removes superoxide we show for the first time that this radical plays a key role in NMDA-mediated hyperalgesia. Intrathecal administration of NMDA in rats led to a time-dependent development of thermal hyperalgesia. Removal of superoxide with M40403 abolished NMDA-mediated hyperalgesia, while its inactive congener had no effect. Thus NMDA-mediated hyperalgesic response to heat is mediated through spinal release of superoxide. At time of near-to-maximal hyperalgesia, we observed that spinal endogenous manganese superoxide dismutase (MnSOD), the enzyme that normally keeps superoxide under well-controlled condition was nitrated, as shown by immunoprecipitation. Subsequently and as determined by biochemical analysis, nitration of MnSOD led to its deactivation as shown by the loss of the enzyme's ability to dismute and hence remove superoxide. M40403 by preventing MnSOD nitration restored its activity and inhibited the hyperalgesic response to intrathecal NMDA. Thus, superoxide-mediated nitration and deactivation of spinal MnSOD is a novel pathway of NMDA-mediated spinal hyperalgesia and hence central sensitization since it helps to maintain high levels of superoxide that in turn maintains nociceptive signaling. The broader implication of our findings is that superoxide may contribute to various forms of pain events that are driven by NMDA-receptor activation.