Abstract Thrombi were produced experimentally in rats and studied at various stages of organization with the electron microscope. Fibrin, which is a prominent constituent of the early thrombus, was virtually absent after 4 days. There was evidence of collagen formation within the wall at 2 days, and there was extensive fibrogenesis within the thrombus at 4 days. The thrombi were first recanalized by solid cell cylinders which were converted into mature capillaries by a process of growth and fusion of extracellular spaces between adjacent endothelial cells. Other significant features and problems relating to the organization of thrombi are briefly discussed.