The value of practolol in the treatment of angina pectoris was studied in three groups of patients by means of different methods. In 23 patients the effect of the drug was evaluated during long-term treatment and in 10 patients in a controlled double-blind trial; in 20 patients, the working capacity was determined by an ECG exercise test on a bicycle ergometer. Long-term treatment revealed advantageous effects of practolol on the incidence of anginal attacks and the number of nitroglycerin tablets consumed in daily life. There was also a noticeable improvement in the ECG. The results obtained in a double-blind trial, with placebo, proved the effectiveness of the drug. The treatment enabled the patients to lead appreciably more active lives. A marked increase in work performance, depending on the dose applied, was confirmed in exercise tolerance tests. No side-effects which would call for discontinuance of the treatment were seen during long-term administration with doses up to 800 mg daily.