Background: Following peripheral nerve injury, rats will show a tactile allodynia and hyperalgesia. But the mechanism of allodynia is still obscure. Previous studies have shown this allodynia was reversed by intrathecal alpha-2 agonists and NMDA antagonists, but not by morphine. In formalin test, either the pretreatment of NMDA antagonist or morphine prevents the hyperalgesia. The present studies, using rats rendered allodynic by ligation of the left L5 and L6 nerves, aimed to investigate the effects of pretreatment of MK-801 and morphine on the development of tactile allodynia. Methods and Material: Male Sprague-Dawley rats (100~150g) were anesthetized with halothane, the left L5 and L6 spinal nerves were ligated tightly by 6-0 black silk. For sham operation muscle dissection was performed but the spinal nerve was not ligated. For pretreatment of drugs, MK-801 (NMDA antagonist; 0.3 mg/kg). CNQX (non-NMDA) antagonist; 0.3 mg/kg), morphine (1 mg/kg) or saline (placebo) was administered subcutaneously 30 minutes before operation. A second dose was administered subcutaneously 24 hours after operation and further doses were given daily for 2 days further. The volume of injection was 5 ml/kg. To assess the mechanical allodynia, paw withdrawal thresholds of ipsilateral limb were determined using 8 von Frey hairs. Results: Within 2 days saline, CNQX or morphine injected rats developed tactile allodynia (paw withdrawal threshold was about 2g), and persisted for over 2 weeks. Pretreatment of MK-801 delayed the development of tactile allodynia for 3 days comparing to that of saline injected rat. Conclusion: NMDA receptor in the central nerve system plays an important role in the development of tactile allodynia induced by peripheral nerve injury. But the mechanism may be different from hyperalgesia developed in formalin test.