We investigated the changes that occur in the shape and the motion of the ventricular septum in experimental right ventricular (RV) infarction with M-mode and two-dimensional echocardiography. The echocardiographic findings were correlated with the hemodynamic alterations. Right ventricular infarction was produced by mercury embolization of the right coronary artery in five anesthetized closed-chest dogs. After embolization, the transseptal end-diastolic left-right ventricular pressure gradient reversed (3 +/- 1) to -1 +/- 1 mm Hg, p less than 0.001). The septal shape was altered by the flattening of the septum at end-diastole and a return to the normal septal shape during systole. Systolic septal motion was reversed after embolization (1 mm toward the left ventricle before embolization to 3 mm toward the RV after embolization, p less than 0.01). Septal thickening was not altered. We concluded that isolated RV free wall infarction results in the reversal of the transseptal end-diastolic pressure gradient and is associated with the flattening of the septum at end-diastole. During systole, the septum returns to its normal shape and moves toward the RV. In addition, systolic septal thickening is preserved. The motion of the septum toward the RV, together with normal septal thickening, may provide mechanical assistance to RV ejection with RV free wall infarction.