1. Ibopamine is a new orally active inodilator, which is developed for the treatment of congestive heart failure (CHF). We treated 13 patients with moderate to severe CHF already on maximal conventional medication with ibopamine and studied its effects on haemodynamic parameters, exercise capacity, plasma norepinephrine (NE) and ventricular arrhythmias on ambulatory ECG monitoring. 2. Both cardiac and stroke volume index increased significantly after ibopamine (33% and 35% resp.). These changes were accompanied by a fall in systemic vascular resistance without changes in heart rate or blood pressure. During cardiopulmonary exercise testing, both maximal oxygen consumption and exercise time increased. Plasma NE levels were not significantly changed after ibopamine. Using the Morganroth criteria for proarrhythmia on ambulatory ECG monitoring, two patients showed proarrhythmia from ibopamine, which was not clinically relevant, but four patients had a decrease in ventricular arrhythmias. 3. This study suggests that ibopamine elicits favourable effects on haemodynamics and exercise tolerance in patients with CHF. During ibopamine treatment, there appears to be no increase in ventricular arrhythmias.