While papillomatous tumors developed in the forestomach of female Ha/ICR mice after a 12-week chronic feeding period of benzo(a)pyrene (BP), no tumors developed in the glandular portion of stomach or in the lung or liver. Among all tissues examined, the forestomach showed the greatest increase of aryl hydrocarbon hydroxylase (AHH) activity following acute or chronic administration of BP. Single acute doses of BP induced AHH activity in forestomach, glandular stomach, lung, and small intestine, but not in the kidney and liver of these animals. Similarly, after chronic administration of BP, AHH activity was inducible in the forestomach, glandular stomach, and lung, but again not in the liver. Although the formation of tumors is associated with greater inducibility of AHH activity in the forestomach after BP administration, the relationship between tissue inducibility of AHH activity and susceptibility to BP carcinogenesis is still not clear. Further studies regarding the formation of specific carcinogenic epoxides of BP in itssues both susceptible (e.g., forestomach) and resistant to BP carcinogenesis would more clearly define the relationship between AHH inducibility and BP carcinogenesis.