This article is commented on by Michel, M. C., pp. 429-430 of this issue. To view this commentary visit http://dx.doi.org/10.1111/bph.13379. Mirabegron is the first β3 -adrenoceptor agonist approved for treatment of overactive bladder syndrome. This study aimed to investigate the effects of β3 -adrenoceptor agonist mirabegron in mouse urethra. The possibility that mirabegron also exerts α1 -adrenoceptor antagonism was also tested in rat smooth muscle preparations presenting α1A - (vas deferens and prostate), α1D - (aorta) and α1B -adrenoceptors (spleen). Functional assays were carried out in mouse and rat isolated tissues. Competition assays for the specific binding of [(3) H]prazosin to membrane preparations of HEK-293 cells expressing each of the human α1 -adrenoceptors, as well as β-adrenoceptor mRNA expression and cyclic AMP measurements in mouse urethra, were performed. Mirabegron produced concentration-dependent urethral relaxations that were shifted to the right by the selective β3 -adrenoceptor antagonist L-748,337 but unaffected by β1 - and β2 -adrenoceptor antagonists (atenolol and ICI-118,551 respectively). Mirabegron-induced relaxations were enhanced by the PDE4 inhibitor rolipram, and the agonist stimulated cAMP synthesis. Mirabegron also produced rightward shifts in urethral contractions induced by the α1 -adrenoceptor agonist phenylephrine. Schild regression analysis revealed that mirabegron behaves as a competitive antagonist of α1 -adrenoceptors in urethra, vas deferens and prostate (α1A -adrenoceptor, pA2  ≅ 5.6) and aorta (α1D -adrenoceptor, pA2  ≅ 5.4) but not in spleen (α1B -adrenoceptor). The affinities estimated for mirabegron in functional assays were consistent with those estimated in radioligand binding with human recombinant α1A - and α1D -adrenoceptors (pKi  ≅ 6.0). The effects of mirabegron in urethral smooth muscle are the result of β3 -adrenoceptor agonism together with α1A and α1D -adrenoceptor antagonism.173415-428