The antinociceptive action of the systemically administered alpha 2-adrenoceptor agonist clonidine was evaluated in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve, using nociceptive tests based on mechanical (vocalization threshold to paw pressure) or thermal (struggle latency to paw immersion in a cold (10 degrees C) or hot (44 degrees C) water bath) stimuli. Experiments were performed 2-3 weeks after surgery when pain-related behavior was fully developed. We demonstrated a dissociative action depending on the test used: clonidine (30-100 micrograms/kg i.v.) had a moderate effect on the abnormal reactions to the mechanical stimulus. By contrast it dramatically increased the struggle latency to hot or cold stimuli. These latter effects were completely prevented by prior administration of the alpha 2-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.).