The influence of the level of carcinogen exposure on histopathological types and cellular differentiation of the induced tumors was examined in 100 male BALB/c mice given N-methyl-N-nitrosourea (MNU) in their drinking water at 240 ppm on alternate weeks (total exposure: five weeks) (groups 1), at 120 ppm similarly (total exposure: ten weeks) (group 2), at 60 ppm for 20 weeks continuously (group 3), or at 30 ppm for 40 weeks continuously (group 4). Forty-three differentiated and 17 undifferentiated type adenocarcinomas were induced. Glandular stomach carcinomas and undifferentiated type lesions were more common in mice treated with a high concentration of MNU for a short period than with a low concentration of MNU for a long period, even though total measured intake of MNU was smaller (P < 0.01). All the induced glandular stomach carcinomas, independent of the treatment schedule, consisted entirely of gastric phenotype cells. In conclusion, the induction of glandular stomach cancers and the proportion of undifferentiated type lesion depend not on the total quantity, but rather on the concentration of the carcinogen, while the phenotypic expression of tumor cells in not affected by the differences in the administration protocol.