BACKGROUND: Activation of glial cells (microglia and astroglia) has been observed in rat neuropathic pain models and they are known to play a role in inducing allodynia in these animals. Minocycline (MC), a tetracycline derivative antibiotics, displays selective inhibition of microglial activation. The author investigated antinociceptive effect of intrathecal administration of MC on muscular pain model induced by repetitive intramuscular injection of low pH saline. METHODS: pH 5 saline was injected into gastrocnemius muscle of rat. Second injection of pH 5 saline was done 3 days after first injection. To study preventive effect of MC, normal saline or MC 50microgram or 100microgram was administered intrathecally 15 min before each low pH saline injection. To evaluate the therapeutic effect, normal saline or MC 500microgram was administered intrathecally after pain model made. To quantify the mechanical allodynia, withdrawal threshold to mechanical stimuli was measured using von Frey filament. RESULTS: Preventive administration of MC 50microgram did not attenuate the mechanical allodynia, on the contrary MC 100microgram administration showed pain attenuation. Therapeutic administration of MC 500microgram did not show analgesic effect. CONCLUSIONS: Preventive intrathecal administration of MC 100microgram attenuated mechanical allodynia, but not in preventive MC 50microgram and therapeutic administration.