Threads impregnated with 20-methylcholanthrene and beeswax, or with pure beeswax, were implanted into the cervical canal and one of the uterine horns of 6-week-old mice. After 4- and 6-weeks’ treatment it was noted that the cervical stratified epithelium extended cranially into the uterine horn containing the thread. Early squamous carcinomas occurred in 58% of the treated animals, and they invariably occurred within an area of benign squamous stratified epithelium, sometimes caudal to the level of the original squamocolumnar junction, sometimes at the level of the original junction, but most frequently cranial to it in the area of newly-formed stratified epithelium. It is concluded that all squamous tumors originated from the cervical stratified epithelium, and that none arose by metaplasia and neoplastic transformation of columnar cells, or from any other component of the uterus.