The effect of therapy with lidoflazine on maximal exercise in the upright position was evaluated in 21 patients with angina pectoris. The study consisted of the following three consecutive periods: (1) a three-month period of receving placebo; (2) six months of therapy with lidoflazine; and (3) a six-month period in which patients were randomized to either therapy with lidoflazine or placebo. Functional status was monitored by multistage tests of exercise capacity and the amount of nitroglycerin consumed. From period 1 to period 2, the mean maxial exercise time increased from 4.4 to 6.5 minutes (48 percent; P less than 0.001), and the external workload increased by 68 percent (P less than 0.001). the mean heart rate at two minutes of exercise decreased from 114 to 101 beats per minute (P less than 0.001) but was unchanged at symptom-tolerated maximal exercise. During period 3, the patients receiving therapy with liodflazine maintained their improved exercise tolerance, and the reduction in mean heart rate at two minutes of exercise persisted. Patients receiving placebo during period 3 had a decrease in exercise tolerance, and the mean heart rate at two minutes of exercise increased to control values. Lidoflazine in effective as an antianginal medication, in part due to suppression of the heart rate during exercise.