The influence of isosorbide dinitrate on hemodynamics and gas exchange was studied in 10 patients with coronary artery disease and 10 patients with chronic cor pulonale. A significant decrease in total pulmonary vascular resistance was found in both groups. This effect seemed mainly to be due to improved conditions for the left ventricle in the patients with coronary heart disease, while it could be explained by vasodilatation of the precapillary pulmonary resistive vessels in the patients with cor pulmonale. In the latter group, there was also a significant increase in the alveolo-arterial PO2 gradient.