Objective To observe the effect of intrathecal agmatine(AG) at the spinal cord level on morphine tolerance. Methods (1) Twenty-four SD rats were divided into four groups: control group, pure intrathecal injection(i.t.) of saline(15 μL); morphine group, i.t. morphine(15 μg/15 μL); AG group, i.t. AG (12.5 μg/15 μL); and morphine+AG group, i.t. morphine (15 μg/5 μg) and AG (12.5 μg/10 μL). Rats of the four groups were injected with 0.2 mg bee venom subcutaneously in the plantar 5 min after i.t, and the numbers of spontaneous paw withdrawal reflex were recorded within 1 hour. (2) Twenty-four SD rats were divided into three groups: control group, pure intrathecal saline(15 μL); morphine tolerance group, i.t. morphine(15 μg/5 μL) twice a day for 4 consecutive days; and morphine tolerance+AG group, i.t. morphine(15 μg/5 μL)twice a day for 4 consecutive days, on the fourth day the rats also received AG(12.5 μg/10 μL). Half of the rats were examined for thermal paw withdrawal latency and mechanical withdrawal threshold after i.t, and the other half was injected with 0.2 mg bee venom subcutaneously in the plantar 10 min after last dose; the number of spontaneous paw withdrawal reflex were recorded within 1 hour. Results (1)Compared with the control group, the numbers of flinches of intrathecal morphine and AG groups were significantly reduced(P<0.05). (2) In intrathecal morphine tolerance model, the thermal paw withdrawal latency and mechanical withdrawal threshold were significantly increased in the morphine tolerance+AG group compared with the control group (P<0.05);morphine tolerance + AG group had significantly stronger inhibitory effect against spontaneous s.c. BV-induced pain, with the number of spontaneous flinches within 1 hour decreased significantly in the morphine tolerance + AG group(P<0.05). Conclusion Intrathecal AG has synergistic effect with morphine in pain relieving, and it can also reverse the morphine tolerance induced by repeated i.t. injection.