Objectives. The purpose of this study was to evaluate left ventricular performance by on-line pressure-area relations using echocardiographic automated border detection in the in situ canine heart in a manner similar to pressure-volume analyses. Background. Echocardiographic automated border detection can measure ventricular cavity area as an index of volume and may be interfaced with pressure to construct pressure-area loops on-line. Methods. Eight anesthetized open chest dogs had simultaneous measurement of ventricular pressure, aortic flow and midventricular short-axis area. Pressure-area loops were constructed by a computer workstation interfaced with the ultrasound system. Stroke area (Maximal area - Minimal area) and stroke force (∝P dA [P = pressure; A = area]) values during inferior vena cava (n = 8) and aortic (n = 4) occlusions were compared with stroke volume and estimates of stroke work, respectively. Inotropic modulation was induced with dobutamine infusion (2 to 5 μg/kg body weight per min), followed by propranolol infusion (2 to 5 mg). End-systolic and maximal elastance and preload recruitable stroke force (stroke force versus end-diastolic area) were derived for each period. Results. Changes in stroke area and stroke force were significantly correlated with changes in stroke volume and estimates of stroke work during caval occlusion (n = 8) (r = 0.87 ± 0.02, SEE = 8 ± 1% and r = 0.90 ± 0.03, SEE = 8 ± 2%, respectively). In dogs with aortic occlusion (n = 4), changes in stroke area significantly correlated with changes in stroke volume for pooled data (r = 0.84, SEE = 8%, y = 1.0x + 3). Ventricular performance increased with dobutamine infusion (n = 7): endsystolic elastance 30 ± 11 to 67 ± 24 mm Hg/cm2 (p < 0.02 vs. control values); maximal elastance 37 ± 11 to 82 ± 26 mm Hg/cm2 (p < 0.02 vs. control values); preload rcruitable stroke force 81 ± 24 to 197 ± 92 mm Hg (p < 0.02 vs. control values). Decreases occurred with propranolol infusion (n = 5) end-systolic elastance 20 ± 4 to 13 ± 4 mm Hg/cm2 (p < 0.002 vs. control values); maximal elastance 29 ± 8 to 15 ± 5 mm Hg/cm2 (p < 0.002 vs. control values); preload recruitable stroke force 66 ± 14 to 40 ± 9 mm Hg (p < 0.002 vs. control values). Conclusions. On-line pressure-area relations are a potentially useful means to assess left ventricular performance in a manner that is quantitatively similar to the predicted responses of pressure-volume relations.