The effect of molsidomine on hemodynamic properties was studied in 10 patients with chronic congestive heart failure in New York Heart Association functional classes III and IV. Ten patients with the same degree of heart failure served as control subjects. All patients were receiving standard therapy with digitalis and diuretics. Administration of a single dose of 4 mg of molsidomine orally in the initial phase resulted in significant decreases of pulmonary artery pressure, pulmonary capillary pressure, and right atrial pressure at rest and during exercise (p less than 0.01 and p less than 0.01, respectively). After long-term oral treatment with 4 mg of molsidomine three times daily over a period of 3 weeks, single dose administration of 4 mg of molsidomine orally again caused significant decreases of pulmonary artery, pulmonary capillary, and right atrial pressure at rest and during exercise (range p less than 0.01 to p less than 0.02). Cardiac output, heart rate, systemic arterial pressure, pulmonary artery resistance, and systemic arterial resistance were essentially unchanged.