The effect of drugs that influence the opioidergic and monoaminergic neuronal systems on the antinociceptive action of tizanidine [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiodiazole] was compared with their effect on the action of clonidine. The potency of the clonidine-induced antinoceptive action was 1.83 and 7.75 times greater than that of tizanidine in the tail-flick and acetic acid-induced writhing tests, respectively. The action of tizanidine and clonidine was completely antagonized by pretreatment with yohimbine, an α₂-adrenoceptor blocker, but not by prazosin, an α₁-adrenoceptor blocker. Other α₁-adrenoceptor blockers, phenoxybenzamine and phentolamine, also attenuated the action of tizanidine and clonidine but the potency of these drugs was less than that of yohimbine. An opioid antagonist (naloxone), drugs influencing the serotonergic neuronal system (p-chlorophenylalanine, 5,6-dihydroxytryptamine, cyproheptadine), drugs influencing the catecholaminergic system (α-methyl-p-tyrosine, diethyl-dithiocarbamate, 6-hydroxydopamine, haloperidol) showed no effect on the action of tizanidine and clonidine. From these results, it appears that α₂-adrenoceptors might be of importance in mediating the tizanidine and clonidine antinociceptive action in the tail-flick test.