The left ventricular (LV) end-systolic pressure–volume relation (ESPVR) is a load-insensitive method for evaluating LV contractility, which needs invasive measurement. Some noninvasive methods substitute peak aortic pressure (P S) for end-systolic LV pressure by assuming there is no difference between these pressures. However, this assumption has not been directly validated. With conductance catheter and dual micromanometers, ESPVRs and the slope (EesLV) were constructed from simultaneous LV pressures (LVP) and volumes, aortic pressures (AOP) and LV volumes (EesAO), and P S and LV end-ejection volumes (V EE) (EesPP-EEV) during preload reduction in 50 subjects. The ratio of steady-state P S over V EE (P S/V EE) was also checked. AOP and LVP displayed differences of 11 ± 6 and −30 ± 12 mmHg at the onset and end-ejection, respectively, and −2 ± 4 mmHg at end-systole. EesAO and EesLV were nearly identical: EesAO = 0.97 ×EesLV + 0.05, r 2 = 0.99. EesPP-EEV correlated with EesLV (EesPP-EEV = 0.57 ×EesLV + 0.61, r 2 = 0.46) but with much more scatter. P S/V EE correlated worst with EesLV. Central AOP can be substituted for LVP to derive EesLV. Other estimation methods yield weaker and poor correlations to directly measured Ees.