Aim: To observe the effect on spinal synapsin in antagonistic process of ketamine to morphine tolerance in mice. Methods: The experiment was carried out in the Laboratory of Anesthesiology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology between June and September 2003. Twenty-four Kunming micewere randomly divided into 4 groups with 6 mice in each: control group [the rats were given subcutaneous injection of saline (10 mL/kg), followed by intraperitoneal injection of saline (10 mL/kg) after 30 minutes], chronic morphine tolerance group [the rats were given subcutaneous injection of morphine (10 mg/kg), followed by intraperitoneal injection of saline (10 mL/kg) after 30 minutes], ketamine 10 mg/kg group [the rats were treated with intraperitoneal injection of ketamine (10 mg/kg) at 30 minutes after the subcutaneous injection of morphine, and the ketamine was diluted to 10 mL/kg before use to be of the same volume as that in the control group], and ketamine 20 mg/kg group [the rats were treated with intraperitoneal injection of ketamine (20 mg/kg) at 30 minutes after the subcutaneous injection of morphine]. The rats in all the groups were administrated for twice every day for 9 continuous days. Pain threshold was estimated as the painful index by measuring withdrawal response to Von Frey filament stimulation every other day (1st, 3rd, 5th, 7th, 9th) at 1 hour after the second administration of drugs. The changes of the spinal synapsin immune reaction positive product in the process of morphine tolerance were detected by immunohistochemical method. Results: No animal died during the experiment, and all were involved in the analysis of results. 1 No significant difference of pain threshold was found at all time points in the control group. In the chronic morphine tolerance group, pain threshold was similar to that in the control group on the 1st and 3rd days and gradually decreased from the 5th day to the 9th day. In the ketamine 10 mg/kg group, the change was almost the same as that in the chronic morphine tolerance group. No differences from baseline were noted in the ketamine 20 mg/kg group and control group at any time point, suggesting that ketamine 20 mg/kg in combination of morphine could partly antagonize the development of morphine tolerance. 2 In the control group, the synapsin positive products in the superficial dorsal horn (lamina I-II) were buffer granules, which distributed in cytoplasm. On the 9th dav after treatment of chronic morphine, the expression of the synapsin positive products in the superficial dorsal horn (lamina I - II) in the chronic morphine tolerance group was obviously denser than that in the control group. The expression of synapsin positive products in the ketamine 10 mg/kg group had no obvious change as compared with that in the chronic morphine tolerance group. The synapsin positive product in the ketamine 20 mg/kg group was significantly sparse as compared with that in the chronic morphine tolerance group. 3 The absorbance value of synaptophysin positive product in the superficial dorsal hom in the chronic morphine tolerance group on the 9th day was remarkably increased by 35% as compared with that in the control group [(97±11), (72±9), P < 0.05]. But that in the ketamine 10 mg/kg group was similar to that in the chronic morphine tolerance group [(93±11), (97±11), P > 0.05], but significantly higher than that in the control group (P < 0.05). The absorbance value of synaptophysin positive product in the superficial dorsal horn in the ketamine 20 mg/kg group on the 9th day was reduced by 20 % as compared with that in the chronic morphine tolerance group [(78±7), (97±l1), P < 0.05], but insignificantly different from that in the control group. So 20 mg/kg ketamine could significantly reduce the absorbance of spinal synaptophysin during tolerance in mice. Conclusion: The up-regulation of spinal synaptophysin may be involved in the mechanism of morphine tolerance, and ketamine can partly restrain the morphine tolerance through up-regulating spinal synaptophysin.