In addition to its many functions in biosynthesis, growth, coagulation and rheology, vascular endothelium is anatomically interposed between the vascular space and the tissue fluid. Recent evidence indicates that it mediates cellular and molecular exchange between these compartments. The exchange can occur through differentiated microdomains of endothelium such as fenestrae. These areas are differentiated with regard to surface charge, protein distribution within the lipid bilayer, membrane fluidity and other features. The exchange is also affected by certain characteristics of the molecule to be transported: molecular size, charge, shape and its carbohydrate content. Proportionately, the largest volume of exchange occurs across the endothelial cytoplasm by vesicular transport systems. Two systems are particularly in evidence; (a) receptor-mediated transcytosis which is specific, and (b) fluid-phase endocytosis. The molecule may become modified in transit and the modification may be of essence in determining its target point and its subsequent metabolism. While most of these modifications involve the carbohydrate moiety of the glycoproteins, glycosylation of non-glycoproteins such as albumin, may also be of physiological significance in transendothelial transport. By virtue of its transport potential, albumin can thus affect the transport of other substances. Recent advances in the molecular transport function of endothelium have been reviewed in the context of its physiological and clinical significance. The basis for the concept of a generalized tissue-blood barrier has been offered.