Oxygen-derived reactive species generated by xanthine/xanthine oxidase (X/XO) can modulate the N-methyl-D-aspartate (NMDA) receptor via its redox-sensitive site. Here it is shown that hydroxyl radicals are the agents responsible for NMDA receptor oxidation by X/XO. Spectrophotometric assays revealed that the amounts of superoxide anion and H2O2 produced by X/XO were not decreased by the hydroxyl radical-specific scavenger mannitol. This sugar, however, could prevent most of the oxidizing actions of NMDA receptors by X/XO, but not by the thiol oxidizing agent 5,5 -dithio-bis-nitrobenzoic acid. Finally, a non-enzymatic source of hydroxyl radicals was also effective in oxidizing the receptor's redox site. Hydroxyl radicals may thus represent the final common pathway for the modulation of NMDA receptor function by oxygen-derived free radical generating systems.