Thirty-three male Sprague-Dawley rats were fed a carcinogenic (0.064% 3'-methyl-4-dimethylaminoazobenzene, 3'-Me-DAB) ground meal normal diet. After 12 weeks the ground meal diet was replaced with a normal pellet diet, and the 30 surviving animals were divided into three equal groups. One group was sacrificed at the twelfth week and the other groups 4 and 8 weeks later. Control animals were also run. Based on previous studies which used "tumor-promoting" diets and 3'-Me-DAB, we expected a less than 100% incidence of predominantly hepatocellular carcinomas. However, we found mucin-producing cholangiocarcinomas in all 30 animals and, in addition, a small hepatocellular component in 3 of the animals. By electron microscopy the intestinal mucosal features of microvillous border cells, goblet cells, and endocrine-like cells were found. We suggest that the tumors produced as described here provide a good animal model of mucin-producing cholangiocarcinomas.