It is demonstrated that negative, "subatmospheric" pressure developing in the heart ventricles during the rapid-filling phase is an evidence of active diastole and inflated capacity of heart chambers. Diastolic activity of the heart increases as a result of insufficient blood inflow to the ventricles coupled with inadequate venous return or narrowed atrioventricular orifice. It is assumed that the heart's diastolic activity is an adaptive-compensatory mechanism adequately stabilizing circulation under changing hemodynamic conditions.