In dogs, isosorbide dinitrate (ISDN) reduced venous return, and in spite of mild tachycardia the cardiac output fell by reduction of stroke volume. Depending on modalities of administration, the drug had little or no influence on systemic arterial pressure. Cardiac inotropism remained unchanged. Heart work was consistently reduced. During temporary occlusion of the left coronary artery, ISDN had little effect on regional blood flow in normal myocardial areas but increased transmural blood flow and the endocardium/epicardium perfusion ratio in ischaemic areas. The redistribution of coronary blood flow produced by ISDN to the benefit of ischaemic areas was accompanied by a simultaneous decrease in the intensity of electric ischaemia.