The early stages of atherosclerosis involve the accumulation of plasma constituents, including fibrinogen, lipoproteins and lipids and their modified forms. To assess the role of haemodynamics in these processes, we have measured the pressure dependence of the fluxes of albumin, fibrinogen and fluid through the whole thickness of the walls of the carotid artery and the inferior vena cava. Fluxes were much higher across the vein, the vessel which is less susceptible to atherosclerosis. Protein fluxes showed a marked non-linear dependence on transmural pressure which was greater than the pressure dependence of the fluid fluxes. Protein movement across the artery wall could be modelled assuming a significant convective component with the protein reflection coefficient decreasing as the wall tissue was stretched by the increasing pressure. Protein movement across the vein wall appeared to be dependent on ultrafiltration with the formation of a concentration polarization layer at the luminal interface; such a layer would be extremely sensitive to flow within the vessel lumen.