Detection of gastric carcinoma in the experimental rat can be difficult. We investigated whether or not serological testing for carcinoembryonic antigen might aid in the diagnostic process. Twenty-five young adult male Wistar rats were studied; 15 were treated with the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and 10 received no treatment. A complete autopsy was done on the 22 animals (12 in MNNG group, 10 in control group) that completed the 37-week study. Serum obtained at autopsy was evaluated by radioimmunoassay for the presence of CEA-like immunoreactivity. Grossly normal stomach, as well as gastric and small bowel cancers, were also stained for CEA. Of the 12 autopsied animals in the MNNG group, eight had a total of 12 cancers (seven gastric and five proximal small bowel); in no animal was an elevated serum CEA level detected. Immunoreactive tissue CEA was demonstrable in normal stomach and in gastrointestinal cancers; the tumors usually stained less intensely than adjacent normal stomach and did not exhibit normal intracellular CEA distribution.