Abstract: Novel, noninvasive in vivo methods and apparatus for measuring the end-systolic pressure-volume relation and deriving peak end-systolic elastance, E.sub.max, which do not alter intrinsic or reflex-stimulated cardiac contractility are disclosed. Afterload is varied by the controlled inflation of an intraaortic balloon catheter positioned in the ascending aorta. Balloon inflation is timed to transiently interrupt ventricular ejection at different times during the ejection phase, producing contraction at different ventricular volumes. Simultaneous measurements of left ventricular pressure and aortic volume during the occlusion sequence allows pressure vs. ejected volume loops to be generated, from which the slope of the end-systolic pressure-volume relationship, E.sub.max is determined.E.sub.max was measured in 6 Dorsett sheep with normal, enhanced, and depressed contractile states. In the normal states, E.sub.max ranged from 1.01 to 5.08 mmHg/ml in animals with body weights ranging from 23 to 32 kg.