Thromboresistance of the arterial wall is the result of many factors which dynamically interact with each other. Glycosaminoglycans (GAG), among which heparin, synthesized by endothelial cells, smooth muscle cells, and fibroblasts, play an important role in determining vessel wall non-thrombogenicity. We are describing morphological-morphometrical characterization (increase in endothelial cell density and nucleus/cytoplasm ratio) of bovine aortic endothelial cells grown in the presence of heparin and other GAGs.