Ventricular filling in the isolated right ventricle was studied. Mechanical impedance of the ventricle was estimated from simultaneous ventricular pressure and inflow measurements. Changes in work load or heart rate were accompanied by reproducible and characteristic changes in impedance and intraventricular pressure-curve contour. These were discussed in relation to changes in ventricular relaxation, elasticity, and viscosity. The right ventricle fills according to a pattern fixed by hemodynamic factors which affect the mechanical properties of the ventricle. The physical state of the ventricle early in diastole appears to be a determinant of stroke output.