Effects of discontinuation of chronic feeding of diethylnitrosamine on the development of hepatomas in adult rats.

Diethylnitrosamine (DENA) at 10 mg/kg was fed to adult rats either continuously or for periods ranging from 1 to 10 weeks. Survival correlated inversely with the duration of carcinogen feeding. Less than 4 weeks of DENA feeding produced only preneoplastic foci that persisted indefinitely; 4 weeks were found to be necessary for the transformation of preneoplastic lesions into liver cancers; after 6 weeks, the incidence of hepatomas was 100%. The process of liver cancerization appeared to be identical whether DENA was fed for 8 weeks or continuously up to the time of death. These results are discussed in the light of the evolution of the homoeostatic control of liver-cell division during DENA feeding, in order to distinguish the different successive roles played by the carcinogen.