Purpose: Tramadol produces a conduction block similar to lidocaine by exerting a local anesthetic-like effect. The aims of this in vitro study were to determine the effects of tramadol on the vasorelaxant response induced by the adenosine triphosphate-sensitive K + (K ATP ) channel opener, levcromakalim, in an endothelium-denuded rat aorta, and to determine whether this effect of tramadol is stereoselective. Methods:The effects of tramadol (racemic, R(-) and S(+): 10 -6 , 10 -5 , 5 × 10 -5 M), and glibenclamide on the levcromakalim dose-response curve were assessed in aortic rings that had been pre-contracted with phenylephrine. In the rings pretreated independently with naloxone, and glibenclamide, the levcromakalim dose-response curves were generated in the presence or absence of tramadol. The effect of tramadol on the dose-response curve of diltiazem was assessed.Results: Racemic, R(-) and S(+) tramadol (10 -5 , 5 × 10 -5 M) attenuated (P < 0.0001) levcromakalim-induced relaxation in the ring with or without naloxone in a dose-dependent manner. The magnitude of the R(-)-tramadol-induced attenuation of vasorelaxant response induced by levcromakalim was greater (P < 0.05) than that induced by S(+)-tramadol. Glibenclamide almost abolished the levcromakalim-induced relaxation. Tramadol, 5 × 10 -5 M, did not significantly alter the diltiazem-induced relaxation. Conclusion: These results suggest that a supraclinical dose (10 -5 M) of tramadol [racemic, R(-) and S(+)] attenuates the vasorelaxation mediated by the K ATP channels in the rat aorta. The R(-) tramadol-induced attenuation of vasorelaxation induced by levcromaklim was more potent than that induced by S(+) tramadol. This attenuation is independent of opioid receptor activation. Méthode : Les effets du tramadol (racémique, R(-) et S(+) : 10 -6 , 10 -5 , 5 × 10 -5 M), et du glibenclamide sur la courbe dose-réponse du levcromakalim ont été évalués sur des anneaux aortiques pré-contractés avec de la phényléphrine. Sur les anneaux indépendam-ment prétraités au naloxone et au glibenclamide, les courbes de dose-réponse du levcromakalim ont été générées en présence ou en l'absence de tramadol. L'effet du tramadol sur la courbe doseréponse du diltiazem a été évalué. Résultat : Le tramadol racémique R(-) et S(+) a atténué (P <