A new method has been devised for preparing artificial cecal pouches lined with mucous epithelium in the lower lips of Wistar and Sprague-Dawley rats in order to make carcinogens act continuously for a long time in the oral mucosa. When a 0.5 per cent mineral oil solution of DMBA, a crystal of MC, and a crystal of NG were administered, squamous-cell carcinoma, carcinoma in situ, papilloma, adenoma sebaceum. neurofibroma, fibroma, hemangiosarcoma, hemangiosarcoma, hemangioma, and lymphangioma were successfully produced in the oral mucosa of rats. In addition, interesting findings were obtained concerning tissue changes in the process of carcinogensis in the mucous epithelium.