Pressure-volume relationship of the left ventricle at endsystole was studied in three different series of experiment on the dog. Intraventricular pressure was measured with a miniature gauge at the apex. Intraventricular volume was accurately measured by three different methods: (1) plethysmographically with a cardiometer cup, (2) with an intraventricular balloon and a volumetric cylinder, and (3) with the same balloon and areciprocal pump which could clamp the endsystolic volume. We found consistently that as long as the contractile state was stable the endayatolic pressure-volume relationship curve was largely independent of the loading conditions such as enddiastolic volume and ventricular afterload pressure and also of whether the mode of contraction was isovolumic, auxobaric or physiologically ejecting. When the endsystolic pressure was kept unchanged in the ventricle with a stable contractile state, the intraventricular volume decreased to the same volume as given by the endsystolic pressure-volume relationship and the afterload pressure. When the endsystolic volume was maintained constant, the ventricle developed at the end of systole almost the same pressure as that predicted from the same endsystolic pressure-volume relationship. The endsystolic pressure-volume relationship, therefore, uniquely characterizes the degree of ventricular compression under a given contractile state.