BACKGROUND: The facilitatory effect of spinal prostaglandins (PGs) on nociceptive transmission suggests that early PG synthesis after nerve injury could be important in the development of allodynia. METHODS: The aim of this study is to examine the effects of diclofenac (nonselective COX inhibitor), SC-560 (selective COX-1 inhibitor), and NS-398 (selective COX-2 inhibitor) on mechanical allodynia and thermal hyperalgesia in the neuropathic pain model. The rats underwent right L5 spinal nerve ligation (SNL) and were assigned to three COX inhibitor groups to be injected intraperitoneally with different administration dosages (0.2 mg, 1 mg, 5 mg) 30 minutes before, and at 1, 2, and 3 days after SNL. The withdrawal threshold of both hindpaws in response to mechanical stimulation was measured by dynamic plantar anesthesiometer and the withdrawal ratio of right to left hindpaw was calculated. The thermal stimulation applied to both hindpaws by the plantar test was calculated different administration dosages were compared with the vehicle group. RESULTS: There were no differences in mechanical allodynia among the lower dosage groups (0.2 mg) until 14 days after SNL. However, 1 mg of NS-398 decreased mechanical allodynia compared with the vehicle group at 14 days after SNL, and 5 mg of NS-398 decreased mechanical allodynia at 3 days after SNL. However, there was no difference in thermal hyperalgesia between the groups. CONCLUSIONS: These results suggest that intraperitoneal administration of COX inhibitor (especially selective COX-2 inhibitor) after nerve ligation injury can attenuate the development of mechanical allodynia.