The effects of a combined alpha and beta receptor antagonist, labetalol, were investigated in 10 patients with chronic stable angina pectoris. The optimal dose was determined during an initial dose titration study when the patients were treated with 200 mg, 400 mg, and 600 mg (six patients) of labetalol a day. The effective dose was then compared with placebo in a double blind randomised study. The effects of the drug were monitored with angina diaries, treadmill exercise testing, and 48 hour ambulatory electrocardiographic ST segment monitoring. Plasma labetalol concentrations were measured during each treatment period. The mean effective antianginal dose of labetalol was 480 (SD 140) mg/day given by mouth twice a day. There was a dose related reduction in daytime and nocturnal heart rate, the frequency of pain was significantly reduced by 41%, and exercise duration was significantly increased by 44% with labetalol when compared with placebo. The frequency and duration of the episodes of ST segment depression were significantly reduced by 56% and 73% respectively with labetalol. Adverse effects resulted in a reduction of the dose of labetalol in two patients. Thus labetalol is an effective agent in the treatment of angina pectoris.