Abstract: Objective:To investigate the relationship between the antinociception of ketamine(Ket) mediated by nitric oxide(NO) pathway and inhibiting substance P(SP) receptor in the spinal cord, and its possible mechanism.Methods:The SP of intrathecal injection and/or Ket of intraperitoneal injection were recorded using behavioral experiment, immunohistochemical staining and spectrophotometry.The incubation period of licking foot reaction in mice and licking time in phase Ⅰ and phase Ⅱ mice were detected using hot-plate test and using formaldehyde experiment, respectively.The changes of the number of Fos-like immunoreactive(FLI) positive cells, nitric oxide synthase(NOS) activity and NO content were measured.Results:In hot-plate test, the 20 mg/kg and 30 mg/kg of Ket of intraperitoneal injection could increase the pain threshold in the hot-plate test(HPPT) of mice(P<0.05 to P<0.01), the 0.5 ng of SP of intrathecal injection after 5 min and 20 min could decrease the HPPT of ketamine-treated mice(P<0.01 and P<0.05).In formalin test, the 30 mg/kg of Ket of intraperitoneal injection could decrease the licking time of mice(P<0.01), the 0.5 ng of SP of intrathecal injection could increase the licking time of mice in two stages(P<0.05 to P<0.01).In control group, the few FLI positive cells were symmetrical distribution on both sides of spinal dorsal horn, the FLI positive cells on both sides of spinal dorsal horn injected by formalin were significantly higher than that in control group(P<0.01).The FLI positive cells on both sides of spinal dorsal horn injected by intraperitoneal Ket significantly decreased(P<0.01), the 0.5 ng of SP of intrathecal injection could significantly inhibit FLI positive cells on both sides of spinal dorsal horn injected by formalin(P<0.01).Ket could inhibit the copy paste in spinal cord(P<0.05), and the SP of intrathecal injection can counter copy paste caused by Ket(P<0.05).Conclusions:The antinociception of Ket is correlation with the inhibiting of SP receptor, which may be mediated by NO pathway.