Abstract: The aim of the present study was to investigate the effects and possible mechanism of tetramethylpyrazine(TMP)on morphine-induced microglia activation and tolerance. The antinociception and morphine tolerance were assessed in mice using hot-water tail flick test. IBA-1(ionized calcium binding adapter molecule 1), the marker of microglia, was detected by immumofluorescence method. The expression of p-p38 MAPK and total p38 MAPK(mitogen-activated protein kinase, MAPK)was analyzed by Western blot; real-time polymerase chain reaction(RT-PCR)was used to detect the expression level of tumor necrosis factor-α(TNF-α)and interleukin-1β(IL-1β). Results showed that TMP(15, 30, 60 mg/kg, ip)inhibited morphine-induced up-regulation of IBA-1, p-p38, TNF-α and IL-1β in a dose-dependent manner, yet with no effect on the expression of total p38 MAPK. In conclusion, TMP significantly inhibited the activation of microglia evoked by morphine via p38 MAPK signaling pathway, thus attenuating morphine antinociception tolerance.