Substantial evidence has established that activation of the NMDA receptor in the spinal dorsal horn is essential for central sensitization-a phenomenon which comprises various pathophysiological mechanisms underlying neuropathic pain-like signs in animal models. In the present study, a partial sciatic nerve ligation in the rat was used to produce a model of nerve injury-induced pain represented by hypersensitivity to innocuous stimuli ("allodynia"). The aim was to assess whether alteration of NMDA receptor expression correlates with the presence of neuropathic signs. Our approach was to compare spinal NMDA receptor subunit expression and especially subunit 1 phosphorylation, assessed with immunohistochemistry and Western blot at late postoperative times, between nerve-injured rats with marked signs of neuropathy in terms of mechanical and cold hypersensitivity and nerve-injured rats that lacked robust behavioral signs of neuropathy. Quantification of receptor expression was based on comparisons between the dorsal horns ispi- and contralateral to the nerve lesion. The phosphorylated NR1 subunit of the NMDA receptor was found to be significantly increased in the ipsilateral dorsal horn in hypersensitive, but not in non-hypersensitive nerve-injured rats. We did not detect any differences in immunoreactivity in any of the non-phosphorylated NR1, NR2A, NR2B, NR2C or the NR2D subunits. These data suggest that phosphorylation of the NMDA receptor 1 subunit is correlated to the presence of signs of neuropathy (stimulus evoked pain-like behavior) and possibly also to persistent pain following nerve injury. This may represent a mechanism involved in spinal sensitization.