We compared the beta-adrenoceptor stimulant actions of dobutamine and (-)-isoprenaline in isolated tissue preparations (atria, trachea, and uterus) from the guinea pig and in chloralose-anaesthetized, vagotomized cats (arterial blood pressure, heart rate, hindlimb perfusion pressure, and soleus muscle contractility). The results obtained in these experiments indicate that, on a dose basis, dobutamine shows little selectivity in producing alpha-, beta 1-, and beta 2-adrenoceptor-mediated effects. In phentolamine-treated cats, reductions in arterial pressure and total peripheral resistance produced by infusions of dobutamine were little affected by the beta 2-adrenoceptor-selective antagonist butoxamine, but were antagonized by atenolol. The rise in cardiac output produced by dobutamine involved increases in both heart rate and stroke volume. There was little indication of a selective inotropic action, a feature that confirmed the results obtained in isolated atrial preparations. The increase in cardiac output appeared to involve both alpha- and beta-receptor-mediated actions, because phentolamine reduced the rise in cardiac output by reducing stroke volume.