Arterial thrombi were induced in rat aorta by a new microsurgical procedure. These thrombi, originally platelet-fibrin masses, got progressively infiltrated by various cell types. Both the importance of mononuclear cells in organization and investment and the origins of foamy cells, smooth muscle-like cells and macrophages, are discussed. Rethrombosis is regularly found at all time intervals. Thrombus volume and its position with respect to the flap are calculated.