Nitrates, predominantly venodilators, are commonly used in the treatment of angina pectoris, whereas hydralazine, a potent systemic and coronary arterial vasodllator, is usually contraindicated. Hydralazine might be useful in therapy of angina H tachycardia could be prevented by beta adrenergic blockade. Consequently, treadmill exercise tolerance was determined in 20 patients with chronic stable angina during treatment with oral propranolol, 60 mg four times daily (control), propranolol plus 20 mg of oral isosorbide dinitrate, propranolol plus 100 mg of oral hydralazine, and all three drugs combined. The mean heart rate at rest in patients recelving propranolol was 54.0 ± 1.7 beats/min (mean ± standerd error of the mean). It increased 6.8 beats/min with the additlon of hydralazine (p < 0.005) and 7.2 beats/min with Isosorbide dlnitrate and hydralazine comblned (p < 0.005), but was unchanged with Isosorbide dinitrarate alone. Systoilc blood pressure decreased 13.5 mm Hg with Isosorbide dinitrate (p < 0.05) and 28.3 mm Hg with Isosorbide dinitrate and hydralazine combined (p < 0.005), but did not change significantly with hydralazine alone (average decrease 4.1 mm Hg). The mean duration of exercise increased by 24.1 seconds with Isosorbide dinitrate (p < 0.05), and almost twice that (42.4 seconds longer than the control value) with Isosorbide dinitrate and hydralazine (p < 0.005). With hydralazine alone, exercise duration was decreased by 24.7 seconds (p < 0.05). Rate-pressure product at symptom-tolerated maximal exercise was unchanged with Isosorbide dinitrarate or hydralazine alone, but lower than the control value with the two agents combined. It Is concluded that Isosorbide dinitrate Improves exercise tolerance in patients with angina who are being treated with propranolol, but hydralazine remains detrlmental despite the use of the beta blocking agent. However, when Isosorbide dinitrate is combined with hydralazine, the detrimental effects of hydralazine on exercise tolerance are reversed.