Acute shifts in the pressure-volume relationship of the left ventricle occur under several conditions. One potential mechanism for this phenomenon is ventricular interdependence, that is, the influence of right ventricular filling on the left ventricle. TTiis mechanism was studied in an isolated cross-circulated ejecting canine heart preparation with balloons sewn into both right and left ventricles and connected to volumetric chambers allowing continuous and accurate simultaneous measurement of right and left ventricular pressures and volume. Seta of left ventricular pressure-volume loops were obtained under a variety of right ventricular volumes. Experiments were repeated with and without the pericardium. Both diastole and systole were studied. We show that in the pericardium-free heart working over a normal range of volumes, the right ventricle exerts little influence on left ventricular pressure. With the pericardium present, there is a small but significant effect of right ventricular volume on left ventricular pressure during systole and diastole. Over a wider range of volumes imposed in the arrested heart, the right ventricle does influence left ventricular pressure even without the pericardium. Thus we have demonstrated that ventricular interdependence is not likely to lead to large acute pressure-volume shifts, either during diastole or systole, except in the presence of considerably above normal right ventricular volumes.