The effect of xamoterol, a beta1 adrenoceptor partial agonist, on resting haemodynamic measurements and exercise tolerance was studied in 10 patients with dyspnoea of effort. All patients had poor left ventricular function due to myocardial infarction with ejection fractions ranging from 15% to 35% (mean 28%). The cardiac index and stroke work index both rose significantly. The mean pulmonary artery pressure fell from 20(2) mm Hg to 16(2) mm Hg and pulmonary artery wedge pressure from 14(2) mm Hg to 10(2) mm Hg within the first four hours. Exercise tolerance, measured on the treadmill, increased significantly in seven patients but was unchanged in the three who had the lowest left ventricular ejection fractions. Exercise heart rate response was attenuated by the drug in all patients. It is concluded that xamoterol may be beneficial in patients with poor left ventricular function but can be harmful in extremely poor left ventricular function where high sympathetic drive may be important.