To examine the existence of pressure equilibrium between tributary veins and the central vena cava during the mean circulatory filling pressure manoeuvre, pressures in the hepatic portal vein, renal vein, and inferior vena cava were determined at 4-s intervals over a 20-s period of circulatory arrest induced by inflating a right atrial balloon in normal blood volume, 10% volume depletion, and 10% volume expansion states in urethane-anaesthetized rats. Portal vein pressure determined 8 s after arrest during volume depletion and expansion was significantly higher than vena caval pressure (6.2 ± 0.8 vs. 3.4 ± 0.2 and 7.7 ± 0.5 vs. 6.2 ± 0.4 mmHg (1 mmHg = 133.32 Pa), respectively; p < 0.01): this pressure disequilibrium continued for 16 s during volume expansion and for the entire 20 s during volume depletion. Renal vein pressure was equal to vena caval pressure during this manoeuvre. Portal vein pressure at normal blood volume was not significantly different from vena caval pressure following circulatory arrest (4.6 ± 0.3 vs. 3.8 ± 0.4 mmHg, respectively). Following ganglionic blockade, portal vein pressure was still significantly higher than vena caval pressure for 12 s during volume alterations. At the 8th s of the arrest the portal pressure determined in volume depletion was 3.6 ± 0.3 mmHg and the inferior vena caval pressure was 2.6 ± 0.4 mmHg (p < 0.05). Under the volume expansion condition, the respective values were 6.5 ± 0.3 and 5.3 ± 0.4 mmHg (p < 0.05). We conclude that, under conditions of blood volume alterations, there is no pressure equilibrium between the portal vein and the inferior vena cava when mean circulatory filling pressure is measured by this technique; a transhepatic barrier independent of reflex control during the measurement of mean circulatory filling pressure appears to play a role in obstructing the establishment of pressure equilibrium within the venous system.Key words: mean circulatory filling pressure, vascular capacitance, hepatic portal vein pressure, unstressed volume.