The role of fibrin in the generation of new blood vessels was examined in this study. Using a wound chamber model, we investigated the sequential interactions between endothelial cells and the extracellular matrix during angiogenesis. Silicone tubes 5 mm long and 1.4 mm in internal diameter were sutured to the cut ends of thigh muscles in the rats. The contents of the chamber were removed at intervals for histological, immunohistochemical and electron microscopic studies. We observed an initial phase of fluid accumulation in the wound chamber followed by formation of a fibrin/fibronectin clot. Migration of endothelial cells, macrophages and fibroblasts into the clot occurred after the 1st week. The subsequent phase of fibrinolysis was accompanied by deposition of collagen and organization of endothelial cells into capillary tubes. These findings support the view that angiogenesis is the product of interactions between endothelial cells and a changing extracellular matrix (ECM) and requires the participation of soluble and immobilized plasma proteins and local ECM factors. Our findings indicate that fibrin is intimately involved in both hemostasis and angiogenesis; these are sequential steps in the initial phase of wound healing. Thus, fibrin/fibrinogen occupies a central position and provides a vital link in the initiation of the cascade event of wound healing.