Short-term coronary ischemia was produced in dogs anesthetized with pentothal by ligating the left anterior descending coronary artery. The resulting changes in representative hemodynamic parameters and the patterns of segmental contraction within and outside the ischemic area were recorded. Two types of reaction to coronary ligation were obtained: in some animals the contractility of the ischemic area rapidly decreased, leading to ballooning of the cardiac wall, inversion of the segmental contraction trace, marked reduction in stroke volume, pronounced increase in ventricular end-diastolic volume, and significant reduction in rate of intra-ventricular pressure rise (dP/dt). In other animals there were no signs of functional ventricular aneurysm, the changes in ventricular end-diastolic volume were less marked, and dP/dt was not altered to a significant extent. These observations suggest that in these cases characterized by the development of a functional aneurysm, the decrease in stroke volume does not result solely from dilation of the entire left ventricle, but also from the increased requirement for systolic fiber shortening in the viable myocardium, which must compensate for the extra stretch in the aneurysmal portion of the ventricular wall. It is suggested that diminution of stroke volume is due not only to the increase in wall stress in the whole left ventricle; it owes also to the expending of contractile elements in the viable portion of the muscle to compensate for additional elasticity in the functional left ventricular aneurysm.