Loops and kinks of the internal carotid, common carotid, and vertebral arteries which were resected in operations on 45 patients were examined histologically and histochemically. Gross pathological changes were found in the arterial wall in the region of the loops and kinks: destruction of the smooth muscles and elastic fibrils and fragmentation of the internal elastic membrane. Fibrous tissue prevailed in all layers of the arterial wall, forming its soft framework and in this way preventing the straightening out of the kinked artery or its loop. The changes were most marked in the loops of the internal carotid artery, microaneurysms with parietal thrombosis were even encountered. The findings of microscopic and histochemical study compel the surgeon to resort to excision of the loop or kink and subsequent establishment of an end-to-end anastomosis or replacement of the whole pathologically changed arterial segment by a graft of autologous vein, as the most radical method of surgical treatment of this pathological condition. In children and in some adults, however, favourable results can be produced by total mobilization of the loop, its downward transposition, resection of its proximal segment, and reimplantation into the bifurcation of the carotid artery.