Objective: The present study was performed to explore the role of mu opioid receptor (MOR) in nociceptive modulation in anterior cingulate cortex (ACC) of rats with inflammatory pain. Methods: To set up an inflammatory pain model, rats received a subcutaneous injection of 0.1 ml of 2% carrageenan into the left hind paw. The hind paw withdrawal latency (HWL) to thermal and mechanical stimulation, by hot plate and Randall Selitto Test respectively, was used to evaluate the rat's responses to noxious stimulation. The influence of inflammatory pain on MOR mRNA level and MOR expression in ACC were assayed by Reverse Transcription Polymerase Chain Reaction (RT-PCR) and western blot. Results: We found that intra-ACC administration of morphine-induced significant antinociceptive effects in a dosedependent manner in rats with inflammatory pain. Furthermore, the antinociceptive effects induced by morphine were attenuated by intra-ACC injection of the opioid receptor antagonist naloxone, indicating involvement of opioid receptor in nociceptive modulation in ACC in rats with inflammatory pain. Moreover, intra-ACC administration of the MOR antagonist β-funaltrexamine (β-FNA) attenuated the morphine-induced antinociception significantly in rats with inflammatory pain. Interestingly, we found that there were significant decreases in MOR mRNA level and MOR expression in ACC in rats with inflammatory pain compared with intact rats tested by RT-PCR and western blot, indicating that there is a down-regulation in MOR expression in rats with inflammatory pain, which support our above results that morphine-induced antinociception was lower in rats with inflammatory pain than that in normal rats. Conclusion: These findings suggest that MOR plays an important role in nociceptive modulation in ACC in rats with inflammatory pain and there is a down-regulation in MOR expression in rats with inflammatory pain.