Physiological role of "suction" in diastolic ventricular filling is still controversial. In order to determine whether the beating intact heart induces negative left ventricular (LV) pressure, indicative of LV suction when the ventricular filling is prevented, we transiently prevented LV filling by occluding the mitral orifice in open chest dogs. In more than 80% of all runs with various hemodynamic conditions the minimal diastolic LV pressure (LVPmin) decreased below zero immediately after the mitral valve occlusion and remained negative throughout diastole. The negativity was augmented with a decrease in end-systolic volume and an increase in inotropic state. The diastolic pressure-volume relationships of the non-filling beating ventricle in control condition and high inotropic state (Ca++ infusion) demonstrated that LVPmin correlated with LV volume and intropic state. These results indicate that the negative diastolic LV pressure is augmented by a decrease in end-systolic volume and by an increase in contractility and thus, LV suction may play an important role in LV diastolic filling.