Simultaneous right and left ventricular function curves (VFC, R, L) were obtained in a canine model, (open chest preparation), with and without pericardium. Preload and afterload conditions for the right and left ventricles were controlled. VFC were constructed from zero to 25 mmHg of ventricular end-diastolic pressures and by increasing the cardiac output from 50 to 250 mL/kg-1min-1. Both, right and left VFC showed an initial steep rise at low filling pressures and then flattened off to a plateau at high filling pressures. The best mathematical model that fitted with the VFC, with and without the pericardium was the parabola (r2 = 0.71, 0.72 respectively). After pericardiectomy R and L VFC were displaced to the left of the VFC with pericardium and a decrease in filling pressures were noted at the same points of cardiac output, findings that suggest a restraining effect of the pericardium. By subtracting the filling pressures obtained with pericardium from those without pericardium at the same levels of cardiac output, pericardial pressures were derived. In all the range of the VFC the pericardial pressures were positive, and this pressure increase as cardiac output increase. Thus the transmural pressure was never cero, for both right and left ventricles. The observed relation for the R and L filling pressures, derived from a polynomial equation of second order suggest a small although not unimportant effect of the pericardium at normal filling pressures, and a very substantial influence at high levels of cardiac output. The demarcation between small and major effects appears in the upper range of normal filling pressures in this dynamic approach of the pericardial pressures.