The effects of the pericardial constraint in control, tamponade, and absent pericardium conditions was studied in 18 anesthetized open-chest dogs. Atrial pressures and systolic and diastolic inflow volumes per beat in the superior (SVC), inferior vena cavae (IVC), and pulmonary vein (PV) were measured. With increasing tamponade, 1) the systolic-diastolic distribution of venous flow became almost exclusively systolic in the SVC (P less than 0.001) and IVC (P less than 0.05), as the gamma-descent disappeared; 2) similar but lesser left-sided changes occurred in PV flows (P less than 0.001) and pressure; and 3) the systolic-diastolic distribution of venous flow was modulated by heart rate. The results imply that during tamponade 1) increased pericardial liquid pressure associated with an increased pericardial constraint couples reciprocal atrial and ventricular volume changes; 2) the atria fill during ventricular ejection (atrioventricular interaction); 3) total heart volume must have been relatively constant throughout a cardiac cycle; and 4) differences in right and left heart compliances may explain persistent diastolic PV flow. Pericardiotomy produced a small increase in the ratio of diastolic to systolic venous inflow volumes (P less than 0.025), suggesting that normally the pericardium has a minor influence on atrioventricular interaction, the volume changes in the atria and ventricles being relatively uncoupled. With severe tamponade, a homogeneous pericardial fluid column tightly couples atrial and ventricular volume changes and accounts for the characteristic changes in atrial pressure waveforms and patterns of venous flow.