Oral mucosal neoplastic disease in rodents has been induced by various chemical carcinogens, including water soluble 4-nitro-quinoline-1-oxide (4NQO). It has been suggested that vitamin C can inhibit, delay or prevent the development of neoplasms, as well as enhance the induction of neoplasia. In this investigation, 4NQO was used to produce a high yield of carcinomas of the palatal mucosa of rats in a relatively short period of time and topical vitamin C was applied to delay the neoplastic transformation. The temporal aspects of the ultrastructural changes occurring in 4NQO-induced oral palatal mucosa treated with both 4NQO and vitamin C have been described and discussed.