The triangular pressure-volume (P-V) area in a P-V diagram bounded by the left ventricular end-systolic and end-diastolic P-V curves and a P-V loop segment during isovolumic relaxation has been suggested to represent an elastic potential energy which has been built during systole (i.e. contraction) and is stored at end-systole (i.e. onset of relaxation) in the ventricular wall. In the present study on canine left ventricles (about 70g), as much as 2/3 of the specific P-V area was found to be actually converted into external mechanical work when ventricular volume was decreased at a relatively slow speed (50–100 ml/s) during relaxation. This amount of external work was about twice as much as that produced by quick release of ventricular volume at 200–300 ml/s at end-systole. The results indicate that the relaxing ventricle can still perform a considerable amount of external mechanical work which cannot entirely be ascribed to the potential energy of non-viscous passive elasticity in the ventricular wall produced as a result of the internal contractile work.