23 patients with acute transmural myocardial infarction were exercised (4 min. 30 Watt) at the end of the hospital period (14-26 days after administration) (probatory exercise). This test was repeated identically after 5 mg of ISDN; again hemodynamic measurements were done (heart rate, arterial pressure, pulmonary artery pressure, cardiac output). 60% of the investigated patients had elevated (above 20 mm Hg) wedge pressures (pulmonary diastolic pressure) during exercise. The elevated PA-pressure was lowered significantly by administration of ISDN. In Group II (with exercise PADP above 20 mm Hg) this reduction of preload improved ventricular performance significantly. In Group I (PADP ex below 20 mm Hg) reduction of the preload worsened ventricular performance (not significant). Heart rate rose at rest and was reduced during exercise after ISDN. Arterial pressure was lower after ISDN as well as during exercise. ISDN proved to be a valuable drug in the postinfarction period even in patients free of angina pectoris since it may help to normalize disturbed hemodynamics (which can only be detected by exercise testing in many cases).