In a previous paper (1) the organization of an experimental arterial thrombus in rat aorta during the first six days was described. The present paper will set out the thrombus organization during the following weeks and months. Within one month, fibrin and platelets inside the thrombus disappear, and are replaced by smooth muscle cells, collagen and elastin. Elastin was deposited in two forms - granular and strand-like, according to exposure to the pulsatile blood flow. In the course of that period the thrombus surface was covered by endothelial cells. Small, non-endothelialized areas, often with adhering thrombi, were found at all time intervals. The occurrence of thrombi of various ages is suggestive of continuing rethrombosis, presumably due to a permanently disturbed haemodynamical situation.