1. We used vessel strips and whole vessels in vitro to determine the length-tension and pressure-volume relationships of two veins with similar diameter and wall thickness, the portal vein and inferior vena cava of rabbits. 2. Length-tension studies indicate that longitudinal and circular strips of the portal vein have significantly smaller elastic moduli than similar strips of the inferior vena cava (P less than 0.05). 3. Pressure-volume relationships of intact vessels indicate that pressures greater than 5 mmHg do not produce significant increases in volume of the inferior vena cava, whereas volume of the portal vein increases significantly over a range of pressures from 2 to 15 mmHg. 4. Activation of smooth muscle with noradrenaline reduces the volume of the vena cava at distending pressures less than or equal to 5 mmHg. In contrast, noradrenaline reduces the volume of the portal vein at pressures up to 10 mmHg. During inactivation in calcium-free solution and activation with noradrenaline, the portal vein is more compliant than the inferior vena cava (P less than 0.05). 5. Morphometric studies demonstrate more collagen in the inferior vena cava than in the portal vein. Differences in capacitance of the vessels may be related, in part, to greater collagen content in the inferior vena cava.