The antinociceptive activity of the intravenously administered alpha 2-adrenoceptor agonists, clonidine and xylazine, was measured in sheep using thermal and mechanical pressure threshold detection systems. Both drugs demonstrated clear antinociceptive activity for both forms of threshold stimuli and clonidine at 6 micrograms/kg i.v. was more potent and longer lasting than xylazine at 50 micrograms/kg i.v. The antinociceptive effects were reversed by idazoxan (0.1 mg/kg i.v.), but were not affected by naloxone at 0.2 mg/kg i.v. indicating that these effects were mediated by alpha 2-adrenoceptors.