Background:Tramadol was recently suggested to be an effective and relatively safe pharmacological treatment for pain and hyperalgesia in urinary colic from calculosis. It can apparently represent a valid therapeutic approach to this medical problem, especially in cases where conventional therapy cannot be applied. However, up to our knowledge, the in vitro effect of tramadol on ureteral smooth muscle contractility has not been investigated. Objectives: The aim of this study is to investigate the effect of tramadol on the ovine spontaneous ureteral activity and attempt to determine its pharmacological basis. Methods: In vitro experiments were performed on ureteral ring preparations in an organ bath. Contractions per minute (frequency) were calculated. The effect of tramadol, was obtained on its own and in the presence of naloxone, chlorpheniramine, phenoxybenzamine, atropine, or diclofenac; while, the effects of histamine, phenylephrine and acetylcholine (ACh) were recorded on their own and in the presence of their respective antagonists. Results: Tramadol (50M) significantly enhanced the spontaneous rhythmic motility (1.21 ± 0.25 to 3.3 ± 0.54). Further, naloxone (2M), chlorpheniramine (M), atropine (M), or diclofenac (10M) failed to inhibit the excitatory effect of tramadol. However, phenoxybenzamine (1M) appreciably attenuated the excitatory effect of tramadol. Conclusions: Tramadol produces substantial excitatory-ureteral activity by a mechanism that is still to be clarified and apparently not dependent on activation of opioid receptors, H1receptors, muscarinic-receptors, or prostanoid synthesis and partly dependent on aminergic mechanisms.