The left anterior descending coronary artery was ligated in 6 baboons. Subsequently, 3 animals were supported with long-term (24-hour) intraaortic balloon pumping (IABP), and 3 were on coronary occlusion alone. Animals were studied hemodynamically and with unipolar electrocardiographic mapping acutely and then were studied after a week and killed. A histological measurement of infarct size was made. The use of IABP had no influence on the area of ischemia determined by unipolar mapping or on infarct size measured quantitatively at a week. Similarly, there were no acute hemodynamic differences between the two groups. The only significant difference noted was a reduction in systolic pressure in IABP animals during balloon pumping and a significantly higher left ventricular systolic pressure a week following infarction in animals treated with IABP. The data indicate no significant effect of IABP on altering infarct size in animals with acute coronary ligation in the absence of cardiogenic shock.