To elucidate direct ventricular interaction, equilibrium radionuclide ventriculography was performed on 18 cases of aortic regurgitation (AR) and 31 cases of mitral stenosis(MS) before and after open heart surgery. Patients with AR were divided into two groups by preoperative left ventricular volume : the normovolumic left ventricle (n-LV) group and the dilated left ventricle (d-LV) group. Patients with MS were divided into two groups by preoperative right ventricular volume : the normovolumic right ventricle (n-RV) group and the dilated rihgt ventricle (d-RV) group. In patients with AR, preoperative right ventricular systolic function in the d-LV group was significantly impaired compared with that in the n-LV group. Postoperative right ventricular systolic function in the d-LV group improved to show no significant difference compared to that in the n-LV group because surgery alleviated the left ventricular dilatation. In patients with MS, preoperative left ventricular systolic and diastolic functions in the d-RV group were significantly impaired compared with that of n-RV group. In d-RV group, postoperative left ventricular systolic and diastolic functions were significantly improved wheh right ventricular dilatation could be relieved postoperatively. In the present study, the existence of direct ventricular interaction influenced by the volume-overloaded ventricle was clarified.