The effects of intraaortic counterpulsation on regional myocardial function were studied using ultrasonic dimension gauges in 24 open chest dogs. Pairs of ultrasonic crystals were implanted in the subendocardium of the left ventricle in control, marginally ischemic and ischemic segments. After coronary arterial occlusion, the end-diastolic length of all three segments was increased. Segment shortening was rapidly replaced by systolic expansion in the ischemic segment. In the marginal segment, active shortening decreased by 53% and, in the control segment, shortening was augmented by compensatory operation of the Frank-Starling mechanism. Balloon pumping initiated 10 minutes after coronary occlusion resulted in an increase in mean aortic diastolic pressure from 103 to 115 mm Hg without any significant change in peak systolic pressure. In the marginal segment, end-diastolic length decreased by 3%, and previously reduced shortening increased to 82% of control value, deteriorating again with discontinuation of counterpulsation. Balloon pumping produced no change in the dyskinetic motion of the ischemic segment but caused compensatory augmentation of shortening of the control segment. Thus, counterpulsation selectively improved segment function of the marginally ischemic segment, presumably as a result of a regional increase in myocardial blood flow and in availability of oxygen due to augmented perfusion pressure. The systolic unloading effect was counterbalanced by improved cardiac performance.