Factors affecting blood movement in the main vessels of the venous system and the changes in pressure and flow values in the vena cava and portal and hepatic veins were simultaneously recorded and related to the phases of the respiratory cycle. The experiments were done in rabbits subjected to nembutal anesthesia, with open abdomen, closed chest and spontaneous respiration. The results indicate an asymmetric pressure gradient in the vena cava centered around the diaphragm with larger pressure differences in the thoracic segment; no pressure gradient in the portal vein, and a sharp gradient of possible functional significance at the caval end of the hepatic veins. Portal, hepatic and abdominal caval pressures were positive, but above the diaphragm, vena cava pressure values were predominantly negative. Flow was continuous, unequally distributed throughout the components and fluctuating alternatively during one respiratory cycle. Maximal pressure and flow values in the portal and hepatic veins were concomitant with the lowest values in the vena cava and closely related to respiration. The distribution of opposite pressure and flow values within the components and their integration during one respiratory cycle suggest that, in the process of venous return, each component and each segment fulfills simultaneously different functions co-ordinated by respiration and cardiac activity.