We prepared a suppository containing 50 mg of N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), and successfully produced experimental colon cancer with good reproducibility by continuous intrarectal insertion of one or two suppositories per day in dogs. The tumors were very similar to human colon cancers, macroscopically and histologically. In one of three dogs subjected to histopathological study, metastases to the lymph nodes, lung, liver and kidney were observed. This animal model produced by a simple procedure will be helpful in investigating treatment of rectal cancer.