The effects of alterations in the diastolic filling pressure of the right ventricle on left ventricular (LV) geometry and filling pressure were studied in six isolated, supported canine hearts. This experimental preparation permitted graded increments in right ventricular (RV) end-diastolic pressure while LV cardiac output, heart rate, and mean aortic pressure were held constant. Endocardial radiopaque markers were placed in the ventricular septum, the anterior wall, the posterior wall, and the free wall in a plane perpendicular to the aortoapical axis. LV end-diastolic dimensions were recorded by x-ray cinematography at 60 frames/sec. The effects of varied RV end-diastolic pressure (0-16 mm Hg) on LV end-diastolic pressure and dimensions were studied at several LV cardiac outputs (780-2880 ml/min) and at several initial LV end-diastolic pressures (1-18 mm Hg). Increments of 5 mm Hg in RV end-diastolic pressure increased LV end-diastolic pressure 2.3 mm Hg. The septum-to-free wall distance decreased by 4.5% from the control distance, but the anterior-to-posterior dimension increased by 4.4%.Thus, LV end-diastolic pressure and LV end-diastolic dimensions were significantly related to RV end-diastolic pressure; LV end-diastolic geometry was increasingly distorted at elevated RV end-diastolic pressure. These data suggest that the high RV filling pressures that characterize certain diseases can secondarily alter LV filling pressures and geometry.