Pressure relationships in the left ventricle of the dog were studied under conditions in which left ventricular inflow was controlled by means of an artificial circuit. Hypothermia was used to slow the dynamics of the heart beat. When all inflow to the left ventricle was suddenly stopped, the succeeding beats showed negative diastolic pressures. The greatest drop in pressure occurred early in diastole; in most instances, end-diastolic pressures remained negative. Early diastolic pressures before occlusion average +2.6 mm. Hg; early diastolic pressures after occlusion of inflow averaged −4.7 mm. Hg. A physical model was proposed which could explain the occurrence and decay of the observed subatmospheric pressure. These experiments indicate that diastolic suction can be a factor that accomplishes ventricular filling during diastole in a heart following systolic discharge against a wide range of aortic pressures.