The rat model of azoxymethane-induced colonic carcinogenesis was evaluated for the occurrence of an adenoma-carcinoma sequence analogous to that in human beings. Male 10-week-old Fischer 344 rats were given 10 weekly subcutaneous injections of azoxymethane at a dose of 15 mg/kg. Every 2 weeks after the first dose, 5 animals were necropsied. A colon tumor was identified first at week 10. At week 16, at least 60% of the rats generally had a colon tumor. The mean size of the tumors tended to increase progressively during the study. Histopathologic examination showed invasion of the muscularis mucosae or deeper layers of the colonic wall in 74% of the tumors. Invasion was unrelated to the size of the tumors, being identified in even the smallest tumors of 1–2 mm of diameter. Invasion was also unrelated to the severity of dysplasia in the tumors, occurring in 83 % of the tumors with low-grade dysplasia, but in 69% of the tumors with high-grade dysplasia. No tumor showed the histopathologic features of an adenoma giving rise to an invasive carcinoma. The findings indicate that an adenoma-carcinoma sequence analogous to that in human beings does not occur in the rat model of azoxymethane-induced colonic carcinogenesi