Physiologists and clinicians commonly refer to "pressure" as a measure of the constraining effects of the pericardium; however, "pericardial pressure" is really a local measurement of epicardial radial stress. During diastole, from the bottom of the y-descent to the beginning of the a-wave, pericardial pressure over the right atrium (P_pra) is approximately equal to that over the right ventricle (P_prv). However, in systole, during the interval between the bottom of the x-descent and the peak of the v-wave, these two pericardial pressures appear to be completely decoupled in that P_prv decreases while P_pra remains constant or increases. This decoupling indicates considerable mechanical independence between the RA and RV during systole. That is, RV systolic emptying lowers P_prv, but P_pra continues to increase, suggesting that the relation of the pericardium to the RA must allow effective constraint, even though the pericardium over the RV is simultaneously slack. In conclusion, we have measured the pericardial pressure responsible for the previously reported nonuniformity of pericardial strain. P_pra and P_prv are closely coupled during diastole but during systole they become decoupled. Systolic non-uniformity of pericardial constraint may augment the atrio-ventricular valve-opening pressure gradient in early diastole and, so, affect ventricular filling.