Recent studies indicate that reactive oxygen species (ROS) play an important role in neuropathic pain, predominantly through spinal mechanisms. Since the data suggest that ROS are involved in central sensitization, the present study examines the levels of activated N-methyl-d-aspartate (NMDA) receptors in the dorsal horn before and after removal of ROS with a ROS scavenger, phenyl-N-t-butyl nitrone (PBN), in animal models of pain. Tight ligation of the L5 spinal nerve was used for the neuropathic pain model and intradermal injection of capsaicin was used for the inflammatory pain model. Foot withdrawal thresholds to von Frey stimuli to the paw were measured as pain indicators. The number of neurons showing immunoreactivity to phosphorylated NMDA-receptor subunit 1 (pNR1) and the total amount of pNR1 proteins in the spinal cord were determined using immunohistochemical and Western blotting techniques, respectively. Hyperalgesia and increased pNR1 expression were observed in both neuropathic and capsaicin-treated rats. A systemic injection of PBN (100 mg/kg, i.p.) dramatically reduced hyperalgesia and blocked the enhancement of spinal pNR1 in both pain models within 1h after PBN treatment. The data suggest that ROS are involved in NMDA-receptor activation, an essential step in central sensitization, and thus contribute to neuropathic and capsaicin-induced pain.