To evaluate the cardiac reserve of the left and right ventricles, equilibrium radionuclide ventriculography was performed on forty patients with valvular heart diseases before and after cardiac surgery. The patients were divided into two groups according to light and electron microscopic findings of the ventricular myocardium obtained during the operations: namely, the preserved group (Group I) and the degenerated group (GroupⅡ). Correlations between the left and right ventricular functions were also studied.In patients with mitral stenosis (MS) and left ventricular volume-overload disease (LVVOD), the preoperative left ventricular function at rest showed no significant difference between Group I and Group Ⅱ, but the left ventricular reserve of Group Ⅱ was significantly deteriorated in comparison with that of Group I.The postoperative left ventricular function at rest showed poor improvement in Group I of MS and LVVOD patients, and the left ventricular reserve of Group Ⅱ was deteriorated in comparison with that of Group I.In preoperative LVVOD patients, a significant positive linear correlation was observed between the left ventricular % ΔEF and the right ventricular % ΔEF. A similar correlation was also observed with % ΔPER(peak ejection rate). Left ventricular volume-overload consequently appears to lead to deterioration of the right ventricular reserve.