Pellets containing 9,10-dimethyl-1,2-benzanthracene (DMBA) were inserted into the submandibular glands of 65 male Fischer rats to determine the nuclear DNA content of both metaplastic epithelium during the process of development of squamous metaplasia (noted at 1, 2, 5, 7, 9, 10, 11, 13 and 15 weeks after insertion) and tumor cells of squamous cell carcinoma 24 weeks after insertion. From the second week of insertion onward, squamous metaplasia was found in excretory duct remaining epithelium in the tissues around the pellet. Dysplasia or a lining of metaplastic epithelium with atypia adjacent to pellets (i.e., formation of an epidermal cyst) was observed 5 weeks after insertion. Coincident with this dysplasia or metaplastic epithelium with atypia, variation in the nuclear DNA content was observed, showing the emergence of octaploids and a widespread histogram pattern with many peaks. Between 7 and 15 weeks, 25%-66% of the lining metaplastic epithelium showed variation in the nuclear DNA content due to shift of a peak (major mode) to a triploid as well as a vague major mode with the appearance of octaploids, etc. Tumors of the submandibular gland were recognized at 24 weeks in 11 (55%) of 20 rats. Nine were keratinized squamous cell carcinomas and the remaining 2 were carcinosarcomas. Marked abnormalities in the nuclear DNA content were observed in 5 of the 11 rats (45.5%), 3 with keratinizing squamous cell carcinoma and 2 with carcinosarcomas. In contrast, no marked variation in the nuclear DNA content was found in 5 rats in which the tumor was associated with cysts.