The mechanical properties (modulus of elasticity and stress-relaxation) of different venous segments of the canine superior vena cava were determined as well as the composition of the vessel wall by means of physical, biochemical and histological methods. It was found that the wall of the vena cava was structurally and mechanically a function of the metric position with respect to the right heart: the modulus of elasticity increased, the stress-relaxation decreased, the concentration of hydroxyproline, collagen and elastin increased and the amount of muscle fibres decreased with increasing distal distance from the right heart. A significant linear correlation coefficient was observed between the modulus of elasticity and the structural wall components. The data presented show the axial heterogeneity and the dependency of the mechanical properties upon the venous vessel wall composition.