Historadioautographic and biochemical studies were undertaken on the transport of 14C-labelled serum very low density and low density lipoproteins (VLDL and LDL respectively) into the aortic wall. Aortas from normal and atherosclerotic rabbits were perfused in vitro with a medium containing VLDL and LDL for 6 hours. Historadioautography revealed the label in all the layers of the perfused aorta. The gradient in the distribution of this label from the inner layers of the aortic wall dawn to the outer part of the media demonstrated that lipoprotein infiltration of the vessel wall is due to uptake of lipoproteins from the aortic lumen. The non-uniform pattern of the distribution of lipoproteins taken up by the atherosclerotic aorta was considered as a result of atherogenesis induced changes in the aortic connective tissue. Both morphological and biochemical studies revealed the increased permeability of the atherosclerotic aortic wall to VLDL and LDL, which appeared to penetrate the walls of blood vessels in the form of intact particles.