BACKGROUND: Clonidine, an alpha2 adrenoceptor agonist, has been known to have antinociceptive and antiallodynic effects. The antinociceptive and antiallodynic effects of brimonidine, a new selective alpha2 agonist, have not been evaluated yet in rats. Behavioral tests were performed to investigate the effects of systemically and spinally administered brimonidine on nociception and mechanical allodynia and the effect of spinal nerve ligation (SNL) on antinociception. METHODS: Rats were prepared with tight ligation of spinal nerves and/or a lumbar intrathecal catheter implantation. Using a hot plate (HP) test or von Frey hair (VFH) test, the effect of intraperitoneal (I.P.) and intrathecal (I.T.) brimonidine in normal and SNL rats were examined. I.P. brimonidine (100 - 1,000 microgram) and I.T. brimonidine (0.1 - 3.0 microgram) were given to examine the antinociceptive effect on an HP test. After a SNL, a HP test was conducted at the same doses of brimonidine to compare with the preoperative state. I.T. brimonidine (0.03 - 3.0 microgram) and saline (control) were administered to examine the antiallodynic effect in SNL rats. In addition, an antagonistic study with yohimbine 1.0 mg/kg I.P. was performed to investigate the reversal of the antiallodynic effect of brimonidine. Allodynic thresholds for lesioned hindpaw withdrawl to a VFH test were assessed and converted to %MPE. RESULTS: I.P. brimonidine produced an antinociceptive effect, and I.T. brimonidine also produced a significant antinociceptive effect (P < 0.05). After an SNL, I.T. brimonidine produced a dose-dependent antinocicpetive effect. In addition, I.T. brimonidine produced a dose-dependent antiallodynic effect which is antagonized by yohimbine (P < 0.05). CONCLUSIONS: The results suggest that brimonidine has a more potent antiallodynic effect when given intrathecally.