Intravenous infusion of inosine (15 mg.kg-1.min-1) to the open-chested pig resulted in hypotension, coronary vasodilatation and slightly increased myocardial contractility. Following coronary occlusion, the action of inosine to increase myocardial contractility was apparently selective. Regional myocardial performance of ischaemic myocardium was increased significantly relative to nonischaemic. The selectivity of inotropic action was not mimicked by glucose-insulin-potassium. It is concluded that the selectivity is multifactoral and that the inotropic, vasodilatory and metabolic actions of the nucleoside contribute to the apparent selectivity.