The influence of the pericardium on cardiovascular phenomena has been examined and evaluated in the light of both ventricular function (modified Starling) curves and pressure-volume relationship of the pericardium. When the left ventricle is stressed, as by an increased aortic resistance, it dilates. The resulting pericardial tension limits right ventricular diastolic expansion and right ventricular work. By this mechanism pulmonary blood volume and pressures are kept at lower levels than would otherwise be present. The pericardium also protects against ventriculo-atrial regurgitation which may account for the observed descending limb of the classically conceived Starling curve.