The carcinogenic process in the liver was monitored from early nonneoplastic changes to the development of tumors in two well-known models; IP administration of dimethylnitrosamine (DMN) to newborn mice, and feeding of rats with acetylaminofluorene (AAF). Liver parenchyma cells and liver tumor cells were isolated by collagenase treatment and prepared for flow cytometric (FCM) measurements of nuclear DNA content. The changes observed were correlated to morphology and 3H-thymidine incorporation. The early preneoplastic changes in hepatocytes from DMN-treated mice were shifts in the DNA content to classes of higher nuclear ploidies. AAF-feeding of rats caused gradual distortion of the DNA content of nuclei from parenchymal cells. After 16 weeks of exposure, before any tumors were seen, a majority of the nuclei displayed an aneuploid DNA content. The significance of these changes in the carcinogenic process is discussed, and a possible use of these models for detection of hepatotropic agents and agents causing aneuploidy (clastogens and turbagens) is proposed. The liver neoplasias (adenomas and hepatocellular carcinomas) induced by these models contained tumor stemlines with diploid DNA content, and some hepatocellular carcinomas showed aneuploidy. Nodules of hyperplasia contained diploid and tetraploid stemline.