The atypical opioid analgesic tramadol has been shown to provide beneficial clinical and urodynamic effects in patients with detrusor overactivity. The effect of tramadol on isolated detrusor muscle has not been studied. This study investigated the ability of tramadol to inhibit acetylcholine (ACh)-induced contractility of the isolated caprine (goat) detrusor muscle. The effect of three concentrations (30, 100 and 300 μm) of tramadol on 10 caprine detrusor strips contracted by the addition of 100, 200 or 400 μm ACh was studied. The sensitivity of tramadol-induced inhibition of ACh responses to treatment with the β-adrenoceptor antagonist propranolol (1 μm) and the opioid receptor antagonist naloxone (100 μm) was also studied. Tramadol caused a concentration-dependent inhibition of ACh-induced detrusor contraction that was reversed by raising the concentration of ACh. Propranolol, but not naloxone, reversed the tramadol-induced inhibition of contractions to ACh in the detrusor. These results suggest that tramadol inhibits ACh-induced contractility of the isolated detrusor. They also suggest that tramadol does so by an indirect anticholinergic mechanism involving the stimulation of β-adrenoceptors. Tramadol may be useful in managing clinical conditions requiring relaxation of the detrusor muscle. Although the concentrations of tramadol needed to relax the detrusor were relatively high, these could be clinically attained via intravesical administration.