The trachea of Syrian golden hamsters was injured by electrocoagulation. The lesions induced as well as the repair process were studied by light and scanning electron microscopy. The changes observed were considered to create the proper conditions for studying the significance of severe focal damage to the respiratory tract for the formation of tumors at the site of injury. Electrocoagulation caused deep crater-like injuries measuring 2 to 3 mm in diameter. Not only the mucosa, but also the submucosal tissues and cartilage rings were damaged. The repair process of the epithelium followed the pattern of covering the defect by squamous epithelium, hyperplasia, and stratified squamous metaplasia and finally differentiation into normal tracheal epithelium. The recovery of the epithelium was strikingly slow; even after a recovery period of 10 weeks regeneration of the epithelium was still incomplete in some animals. This relatively slow process of repair was ascribed to the depth and severity of the injuries involving a large mass of damaged tissues other than epithelium. Regeneration of the cartilage rings comprised three processes, viz removal of necrotic cartilage, growth of fibroblasts into the necrotic cartilage, and deposition of new cartilage along the necrotic rings.