The antinociceptive effects of dexmedetomidine, a highly selective new alpha 2-adrenoceptor agonist, were evaluated in rats after intrathecal, intraperitoneal and subcutaneous administration. Antinociception was tested using the tail-flick method. Both 3 and 6 micrograms of intrathecal dexmedetomidine produced maximal antinociception within 10 min. The effect lasted for up to 6 hr. The smaller dose of 1.5 micrograms produced a mean antinociception of 50% (of the maximum possible effect, MPE%) which lasted for about 2 hr. Subcutaneous atipamezole, a specific alpha 2-adrenergic antagonist completely abolished the antinociception produced by intrathecal dexmedetomidine. When given intraperitoneally, dexmedetomidine produced on average a 50% antinociceptive effect with the highest dose of 60 micrograms/kg. The lower doses of 10 and 30 micrograms/kg were ineffective. After subcutaneous administration a maximal effect was achieved with 120 micrograms/kg, a 70% effect, on average, with 60 micrograms/kg and a short lasting effect of 60% with 30 micrograms/kg. In conclusion, dexmedetomidine is a very potent antinociceptive agent when given intrathecally to rats.