Sequential endoscopic observation of dog colons was performed during colon carcinogenesis. Two beagle dogs were given suppositories containing N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) every day for five months. In month 3, aberrant crypt foci (ACF), a putative preneoplastic lesion, were found in the colons of both dogs, but not in an untreated dog. The frequency of ACF increased until month 10, and then decreased. In month 9, very small lesions, less than 1 mm in diameter, which were similar to human early flat tumors, were first noticed. One of these lesions grew to about 7 mm in size without a change in its shape for 10 months. There were more than ten flat-type tumors in the two dogs, but such lesions were not found in the untreated dog. By biopsy, two of the lesions were proved to be well-differentiated adenocarcinomas histologically. Four polypoid lesions were found in one of the carcinogen-treated dogs. Thus, flat-type adenocarcinomas were induced in the dog colon by ENNG, and their development was followed by magnifying endoscopy.