Dietary habits have been causally implicated in gastric carcinogenesis, whereas minor dietary items may also play a part. Wasabi is a popular pungent spice in Japanese meals. In this study the effect of wasabi on N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced gastric carcinogenesis was studied in rats. Wistar WKY male rats received drinking water containing 50 micrograms/ml of MNNG or tap water alone and a basal diet (PCE-2) or PCE-2 containing 10% (wt/wt) of wasabi powder for 40 weeks. Thus, three groups were completed as MNNG + PCE-2 (n = 30), MNNG + wasabi (n = 30), and tap water + wasabi (n = 30). At autopsy, nine rats (30%) had seven glandular stomach tumors (2 adenocarcinomas, 2 adenomatous polyps, and 3 adenomatous glandular hyperplasias) and three duodenal adenocarcinomas in the MNNG + PCE-2 group, whereas in the MNNG + wasabi group, two rats (7%) had one forestomach epidermoid cyst and one duodenal carcinosarcoma (corrected chi 2 = 4.63, p less than 0.05 for incidences of glandular stomach tumors between 2 groups). In addition, two rats had microscopic atypical glands in the MNNG + PCE-2 group. There was no tumor in the tap water + wasabi group. These results indicated that glandular stomach carcinogenesis induced by MNNG was suppressed by the administration of wasabi.