The effect of systemically administered medetomidine, a selective alpha-2-adrenoceptor agonist, was studied by electrophysiological recordings of the peripherally evoked responses of three different types of sensory neuronal populations in the rat: medial thalamic neurons exclusively responding to mechanical cutaneous stimuli at noxious intensities, spinothalamic tract neurons of the spinal cord responding exclusively or differentially to mechanical cutaneous stimuli at noxious intensities, and low-threshold mechanoreceptive spinal dorsal horn neurons with ascending projections. The neuronal effects were compared with the behavioral data obtained in mechanically and thermally induced nociceptive tail reflex tests in intact and spinal rats. A reversal of the antinociceptive effects was attempted by systemically (1.5 mg/kg, i.p.) or intrathecally (25 micrograms) administered atipamezole, a selective alpha-2-adrenoceptor antagonist. Systemically administered medetomidine produced an atipamezole-reversible, dose-dependent suppressive effect on the evoked responses of nociceptive medial thalamic and spinothalamic tract neurons. A lower dose of medetomidine was needed to suppress significantly (half-maximally) evoked responses of the nociceptive medial thalamic neurons (100 micrograms/kg) than those of the nociceptive spinothalamic tract neurons (300 micrograms/kg). The decrease of evoked responses of the nociceptive spinothalamic tract neurons was accompanied by a decrease in spontaneous activity. The responses of the low-threshold mechanoreceptive projection neurons of the spinal cord were not influenced by medetomidine (30-300 micrograms/kg). The reflex studies with a (anesthetic) medetomidine dose of 300 micrograms/kg indicated that in intact and otherwise drug-free rats, medetomidine produced a significant prolongation of the nociceptive reflex response latency to a tail-pinch and heat; these antinociceptive effects of systemic medetomidine were reversed by systemically and intrathecally applied atipamezole. In spinal rats systemically applied medetomidine (300 micrograms/kg) also produced a significant prolongation of the tail-flick latency, which was reversed by systemically applied atipamezole. The results suggest that a high anesthetic dose of systemically applied medetomidine (300 micrograms/kg) can suppress nociceptive sensory neuronal and reflex responses due to spinal segmental mechanisms through an action on alpha-2-adrenoceptors. This spinal effect is selective to responses of nociceptive neurons, and at least partly postsynaptic as indicated by the concomitant decrease in spontaneous activity. At a lower, subanesthetic (but sedative) dose (100 micrograms/kg) the antinociceptive effect of systemically applied medetomidine can be explained by supraspinal alpha-2-adrenergic mechanisms.