In 8 asthmatic patients a comparative study was performed of the ventilatory and haemodynamic effects of the beta 1-receptor stimulator prenalterol and the beta 2-receptor stimulator terbutaline infused in increasing doses after a placebo. Terbutaline caused a dose-dependent decrease in diastolic blood-pressure (BP) and an increase in systolic BP and heart-rate (HR), while mean arterial pressure (MAP) did not change. Prenalterol produced a dose-dependent increase in MAP and systolic BP, while diastolic BP was unaffected. HR was increased only by the largest dose of prenalterol. The haemodynamic effects of the terbutaline infusion can be explained by a reflex response to the vasodilatation induced by stimulation of the vascular beta 2-receptors, while the effects of prenalterol can mainly be accounted for by a direct action on beta 1-receptors in the heart. These observations show that the cardiac side-effects of beta 2-agonists cannot be avoided by producing more selective agonists. Terbutaline caused a dose-dependent increase in the ventilatory indices. Prenalterol in larger doses caused a limited but significant increase in the ventilatory indices, comparable to the decrease in ventilation caused by the beta 1-selective blocker metoprolol. These findings suggest that the ventilatory effects of metoprolol and prenalterol are mediated via beta 1-receptors in the airways, which apparently play a functional role in asthma.