The possible existence of atypical beta-adrenoceptors in vascular smooth muscle of rat isolated thoracic aorta was investigated. Isoprenaline (10(-8)-10(-4) M) produced concentration-dependent relaxation of phenylephrine (10(-5) M) precontracted rings of endothelium-denuded rat aorta in vitro. Isoprenaline-induced relaxation was resistant to blockade by atenolol (10(-6) M). But, propranolol (2 x 10(-7) M) caused a non-competitive inhibition and SR 59230A (6.6 x 10(-6) M), a beta3-adrenoceptor selective antagonist, failed to produce additional antagonism in presence of propranolol. BRL 37344 (10(-8)-10(-4) M), a beta3-selective agonist, did not relax ring segments precontracted with phenylephrine (10(-5) M) in the absence of endothelium. The non-conventional partial agonist (-)-cyanopindolol (5 x 10(-6)-10(-4) M) induced a marked relaxation in phenylephrine (10(-5)M) precontracted aortic rings without endothelium. This vasodilation was resistant to blockade by propranolol (2 x 10(-7) M) and SR 59230A (10(-5) M). Salbutamol (10(-8)-10(-4) M) produced concentration-dependent relaxation in isolated endothelium-denuded aortic rings precontracted with phenylephrine (10(-5) M). Propranolol (2 x 10(-7) M), but not atenolol (10(-6) M), inhibited this relaxant response. It is concluded that in endothelium-denuded thoracic aorta, salbutamol acts through beta2-adrenoceptors whereas isoprenaline seems to activate both beta2-adrenoceptors and an atypical beta-adrenergic receptor. This atypical beta-adrenoceptor is distinct from putative beta3-adrenoceptor and maybe resembles the reported fourth cardiac beta-adrenoceptor.