Based on the work of Herrold (Oral Surg 25: 262-272, 1968), Thomas and Bollmann (Experientia 25:50-51, 1969), and Leaver et al (Br J Cancer 23:177-187, 1969), we tried to find an optimal pH for N-methyl-N-nitrosourea (NMU) to induce odontogenic neoplasms in rats that were 45 days old at the start of the experiment. Twenty-six Wistar female rats that weighed 75 to 90 gm each were separated into three groups. In group 1, ten rats received a single intragastric dose of 90 mg NMU per kilogram body weight (adjusted to pH 4). in group 2, ten rats received the same treatment, except that the pH was 7. In group 3, six rats were untreated controls. The rats were fed laboratory chow and water ad libitum. After ten months, fibrous neoplasms developed in the anterior part of the mandible of two rats from group 1. The neoplasms were oval and about 4 by 3 mm. Radiographs showed an expansile lesion, which seemed to be in continuity with the mandibular incisor (Fig 1). Regions of calcification were prominent throughout the tumor mass and there was evidence of partial destruction of alveolar bone and regions of tooth resorption. Histological examination showed an apparently keratinized complex odontoma, which was similar to those described in humans. The mass was partially in the periodontal membrane (Fig 2); the greatest extension of the tumor was into the oral vestibule (Fig 3). Neoplasms did not develop in any of the rats in groups 2 and 3. NMU adjusted to a pH of 4 seems to act on the odontogenic epithelium and the gingival epithelium. The gingival epithelium reacquires properties characteristic of the dental lamina, in view of the formation of new "dental laminas" capable of induction of dentinoid tissues and denticles. Gingival epithelium of the rat may be the origin of keratocysts, as well as of regions of ghost cells, masses of dentinoid tissue, and denticles. These results agree with those of Herrold (1968).