Objective: The aim was to examine how regional variations in pericardial pressure affect the mechanical coupling between the ventricles. Methods: Canine hearts from 14 dogs (14.5-18 kg) were removed and placed in cold cardioplegia solution. Balloons were inserted into the left and right ventricles and the atria. Pericardial pressure over the left ventricle (Pclv) and the right ventricle (Pcrv) was measured with thin balloon catheters. Ventricular and pericardial pressures were measured, and ventricular and pericardial coupling was calculated, under control conditions and with increases in pericardial tension and fluid. Results: At baseline, regional differences in pericardial pressure occurred [Pclv>Pcrv, 4.0(SD 0.9) v 2.9(0.6) mm Hg, p < 0.05]. Ventricular coupling via the pericardium was defined as δPclv/δPcrv for right ventricular volume increases and δPcrv/δPclv for left ventricular volume increases. This ratio increased more after increasing right ventricular volume than after increasing left ventricular volume [δPclv/δPcrv>δPcrv/δPclv, 1.14(0.33) v 0.51(0.15), p < 0.05]. Increasing the pericardial tension by clamping the pericardium increased pericardial pressures, yet did not alter the regional variations in pressure [Pclv>Pcrv, 8.4(2.2) v 6.4(2.5) mm Hg, p < 0.05] or pericardial coupling [δPclv/δPcrv>δPclv/δPcrv, 1.18(0.46) v 0.54(0.16), p < 0.05]. In contrast, creating a mild tamponade increased pericardial pressures, eliminated regional differences in pressure, and altered the coupling between ventricles [δPclv/δPcrv≈δPclv/δPcrv, 0.95(0.11) v 1.05(0.08), p=NS]. These regional differences in pericardial pressure might have a geometrical basis. In four in vivo canine experiments using cine magnetic resonance, the short axis radius of curvature for the right ventricle was greater than for the left ventricle [38.3(4.4) mm v 29.2(3.8) mm, p < 0.05]. Conclusions: The pericardium partially protects right ventricular filling: regional differences in pericardial pressure normally occurred with lower pericardial pressure over the right ventricle, and left to right ventricular coupling was less. This protection of right ventricular filling was lost with even a small pericardial effusion.Cardiovascular Research 1993; 27:1470-1476.