The role of different glycosaminoglycan species from the vessel walls as physiological antithrombotic agents remains controversial. To further investigate this aspect we extracted glycosaminoglycans from human thoracic aorta and saphenous vein. The different species were highly purified and their anticoagulant and antithrombotic activities tested by in vitro and in vivo assays. We observed that dermatan sulfate is the major anticoagulant and antithrombotic among the vessel wall glycosaminoglycans while the bulk of heparan sulfate is a poorly sulfated glycosaminoglycan, devoid of anticoagulant and antithrombotic activities. Minor amounts of particular a heparan sulfate (< 5% of the total arterial glycosaminoglycans) with high anticoagulant activity were also observed, as assessed by its retention on an antithrombin-affinity column. Possibly, this anticoagulant heparan sulfate originates from the endothelial cells and may exert a significant physiological role due to its location in the interface between the vessel wall and the blood. In view of these results we discuss a possible balance between the two glycosaminoglycan-dependent anticoagulant pathways present in the vascular wall. One is based on antithrombin activation by the heparan sulfate expressed by the endothelial cells. The other, which may assume special relevance after vascular endothelial injury, is based on heparin cofactor II activation by the dermatan sulfate proteoglycans synthesized by cells from the subendothelial layer.