The effects of xamoterol on exercise capacity have been evaluated in 10 patients with angina pectoris and well-preserved left ventricular function. Compared to placebo a single 200 mg dose of xamoterol produced a slight but insignificant increase in exercise capacity. At maximum work load, ST-T segment depression was reduced (3 mm and 2.19 mm after placebo and xamoterol, respectively; p less than 0.05). The changes paralleled an insignificant reduction in maximal heart rate and pressure-rate product. At rest, xamoterol increased the heart rate from 80 to 86 beats/min. Thus, at rest, when sympathetic tone is low, xamoterol acts as a beta-receptor agonist but during exercise, when sympathetic tone is high, xamoterol acts as an antagonist and reduces myocardial ischaemia.