The effect of intraaortic balloon counterpulsation (IABP) was investigated in normal dogs and after production of ischemic myocardial damage of various extent. Pharmacologic isolation of the heart and ligation of the left anterior descending coronary artery at different levels produced ischemia of the distal third (preparation type 1), distal half (preparation type 2), and distal two-thirds (preparation type 3) of the left ventricular fee wall. The degree of left heart failure (LVF 1 - 3) correlated with the extent of the ischemic damage and was defined by characteristic hemodynamic and metabolic parameters. IABP was most effective when the ischemic damage of the left ventricle was limited and counterpulsation was instituted before significant heart failure developed. When the distal half of the left ventricular free wall became nonfunctional hemodynamic stabilisation remained pump-dependent; to improve survival immediate revascularization of the ischemic area is required. With extensive loss of functional myocardium IABP could merely delay irreversible cardiogenic shock. The experimental data suggest that IABP should be instituted before the blood pressure fall exceeds 25% of its original level. At the stage of profound cardiogenic shock IABP cannot improve survival significantly.