The effects of indoramin, a selective alpha 1 adrenoceptor antagonist, on exercise tolerance were studied in 15 patients using a random double blind crossover protocol. All patients had chronic stable angina, for which 13 had been receiving beta adrenoceptor blocking drugs and nitrates, and these were continued unchanged throughout the study. An initial open challenge with indoramin in eight patients with chronic stable angina showed no serious adverse effects. In a dose of 25 mg three times daily indoramin prolonged exercise duration and increased oxygen consumption during exercise, while the maximal double product was unchanged. The increased exercise capacity was associated with a reduction in ST segment depression during exercise. Side effects included failure of ejaculation in one patient and tiredness in five. These results indicate than alpha 1 adrenoceptor blockade may provide useful additional benefit to patients with stable angina pectoris who are already receiving beta blockers and nitrates.