OBJECTIVE The aim was to determine the contributions of diastolic pressure to the shape of the relationship of total systolic left ventricular pressure with volume (pressure-volume relationship). METHODS The pressure-volume relationship was approximated (by least squares fit) to a parabola P = aV2 + bV + C. Non-linearity was indicated by values "a" significantly different from zero. Negative values indicated concavity to the volume axis, positive values convexity to the volume axis. METHODS Langendorff perfused rabbit hearts (n = 8) with intraventricular balloon were used. Balloon pressure was measured for varying balloon volumes. RESULTS The total systolic pressure-volume relationship was concave towards the volume axis at 2.4 mM extracellular calcium ions concentrations ([Cae++]) a = -47.2 (SD 5.4), p less than 0.05. It was nearly linear at [Cae++] = 0.6 mM; a = -0.8(5.8), p greater than 0.05. It was convex at [Cae++] = 0.3 mM; a = 25.3(4.0), p less than 0.01. The diastolic pressure-volume relationship was always convex: a = 30.1(6.7), 33.5(7.6), 42.2(6.6) for [Cae++] = 2.4, 0.6, and 0.3 mM respectively. When these diastolic values were subtracted from the total pressures, pressure-volume curves for developed pressure were obtained which were always concave: a = -76.9(10.2), -33.5(3.7), -16.3(2.9) for [Cae++] = 2.4, 0.6, and 0.3 mM. CONCLUSIONS The true systolic pressure-volume relationship of the left ventricle is not linear but concave to the volume axis. The slope is therefore variable and not an index of contractility. Apparently linearity or convexity is due to inappropriate addition of the diastolic pressure-volume properties.