The aim of this work was to develop an animal model of accessible colonic tumors simulating human carcinogenesis. A segment of the colon was exteriorized to the skin in 60 rats and swabbed with N-methyl-N-nitrosourea. All the animals developed tumoral abnormalities or frank dysplasic tissue 7 months after the beginning of the experiment. This animal model which closely mimics human colonic carcinogenesis is particularly interesting because tumour development can be monitored in live animals without interrupting normal intestinal functions.