Rapid development of hepatocellular neoplasms in aging male C3H/HeNCr mice given phenobarbital.

One hundred and nineteen male C3H/HeNCr mice, 12 months of age, with spontaneous preneoplastic and neoplastic hepatocellular lesions were given phenobarbital (PB) at 500 ppm in drinking water. Groups of 9-10 mice were sacrificed after 12, 24 and 36 weeks of PB exposure. Identical numbers of untreated controls were used. A group of 6-week-old C3H/HeNCr mice were also given PB and sacrificed at 12, 24 or 36 weeks. In aging mice, PB exposure significantly increased the number of gross tumors or microscopic foci, adenomas or carcinomas per liver at all time periods, especially unique eosinophilic proliferative lesions, while young mice did not develop any focal proliferative lesions by 36 weeks. These findings suggest that in aging mice a fraction of the hepatocyte population (normal, spontaneously-initiated or preneoplastic) is more highly susceptible to phenobarbital 'carcinogenesis' than are hepatocytes of younger mice.