BACKGROUND: There has been much interest in the involvement of wind-up in the hyperalgesia and allodynia of chronic pain syndrome. To investigate the behavioral parallels of this wind-up phenomenon, we evaluated the effects of intrathecal morphine, clonidine and MK801 on the formalin test in the rats. METHOD: All experimental animals were divided randomly into six groups. In group 1(n=7), normal saline 50 l was administered through the intrathecal catheter(PE10, 8 cm in length). In group 2(n=7), morphine 10 g was administered through the catheter. In group 3(n=7), group 4(n=7), group 5(n=7) and group 6(n=7), clonidine 10 g, clonidine 20 g, MK801 1 g, MK801 10 g was administered in same manner, respectively. Thirty min after recoverying from anesthesia, 50 l 5% formalin was injected in the hind paw. To quantify the formalin response, we counted the number of spontaneous flinching for 60 min. RESULT: Intrathecal morphine(group 2) decreased the number of paw flinches in both phases 1 and 2 readily compared to control(p<0.05). In the clonidine(group 3 and 4), both groups decreased the number of flinching in both phases 1 and 2, but there was no significant difference between two groups. In group 5, intrathecal MK-801 decreased the number of paw flinches in phase 2. In group 6, intrathecal MK-801 decreased the number of paw flinches in both phases 1 and 2. CONCLUSION: This study suggest that wind-up is readily blocked by pretreatment of morphine, clonidine and MK801.