We have studied the changes in cell population kinetics and DNA-content of cycling parenchymal cells during the very early steps of rat hepatocarcinogenesis in Faber's protocol. Adult rats were initiated by a single dose of diethylnitrosamine (DENA, 200 mg kg-1), followed 2 weeks later by a 2-week diet of 0.03% 2-acetylaminofluorene (2-AAF) as selection phase. In the middle of selection time, a single necrogenic dose of carbon tetrachloride (CCl4, 2 ml kg-1) was administered by gavage. Twenty four hours thereafter, radiolabelled thymidine (3H-TdR, 1.5 microCi g-1) was given by repeated injections during 24 h. An emergence of small, pyroninophilic ('tigroid') foci was observed at the second, fifth and eighth days after the proliferative stimulus. The focal putative precancerous cells presented a significant higher labelling index (L1) than the non-affected parenchymal cells for all exposure times. However, the labelling intensity decreased from the second to the eighth day after CCl4, suggesting a dilution of the radiolabelled DNA by repeated divisions within the foci. The nuclei of the same foci were analysed for DNA-content by feulgen microdensitometry on neighbouring sections. A gradual reduction of nuclear DNA-content was observed in 66% of the foci at the fifth day and in 100% of foci at the eight day, as compared to surrounding tissue and untreated animals, where labelling and DNA-content remain in the same ratio.