In seven conscious, chronically instrumented dogs, left ventricular volume was calculated with an ellipsoidal model from the anteroposterior, septal-free wall, and base-to-apex left ventricular dimensions, measured by implanted ultrasonic transducers. Matched micromanometers measured left and right ventricular transmural and transseptal pressures. Ventricular pressures and volumes were varied by inflation of implanted vena caval and pulmonary arterial occluders. When compared with vena caval occlusion at matched left ventricular end-diastollc volumes, graded pulmonary arterial occlusions were associated with higher right ventricular systolic pressures, reduced left-to-right transseptal systolic pressure gradients, and leftward systolic septal displacement, with increased septal-free wall segment shortening (all p<0.05). Graded pulmonary arterial occlusions, like vena caval occlusions, reduced left ventricular end-diastolic volume, but left ventricular stroke work at a given end-diastolic volume was greater during pulmonary arterial occlusions (2,674 ± 380 10−3erg) than during vena caval occlusion (l,886 ± 450 10−3erg, < 0.05). These data indicate that, while transient pulmonary arterial occlusion reduces left ventricular preload, the concomitant increase in right ventricular systolic pressure, which is the pressure external to the interventricular septal segment of the left ventricle, augments septal shortening and assists left ventricular pump function at a given preload through direct systolic ventricular interaction. {Circulation Research 1989;65:135–145)