Abstract:Pain threshold (PT) in left foot of rats was significantly increased when a non competitive NMDA receptor antagonist, ketamine, was intraperitoneally injected after the PT was significantly decreased by carrageenin injection; The increase in value of PT became even more apparent and prolonged if both carrageenin and ketamine were injected simultaneously. The results indicate that NMDA receptor seems to be involved in the formation and development of inflammatory pain; The analgesic effect of its antagonist, ketamine, might be improved and the development of inflammatory pain in skin slowed down if the ketamine is given prior to carrageenin injection.