In order to investigate whether the stimulation of liver DNA synthesis might be used to detect one class of hepatic tumor promoters, the incorporation of orally administered radiolabelled thymidine into liver DNA was determined in rats and mice 24 h after a single oral gavage of test compounds at various dose levels. Three DNA-binding hepatocarcinogens, aflatoxin B1, benzidine and carbon tetrachloride, did not stimulate but rather inhibited DNA synthesis (not for CCl4). Four hepatic tumor promoters, clofibrate, DDT, phenobarbital and thioacetamide, gave rise to a stimulation in a dose-dependent manner. Single oral doses between 0.02 and 0.3 mmol/kg were required to double the level of thymidine incorporation into liver DNA (= doubling dose, DD). Differences between species or sex as observed in long-term carcinogenicity studies were reflected by a different stimulation of liver DNA synthesis. In agreement with the bioassay data, aldrin was positive only in male mice (DD = 0.007 mmol/kg) but not in male rats of female mice. 2,3,7,8-TCDD was positive in male mice (DD = 10(-6) mmol/kg) and in female rats (DD = 2 X 10(-6) mmol/kg) but not in male rats. The assay was also able to distinguish between structural isomers with different carcinogenicities. [alpha]Hexachlorocyclohexane stimulated liver DNA synthesis with a doubling dose of about 0.2 mmol/kg in male rats whereas the [gamma]-isomer was ineffective even at 1 mmol/kg. So far, only one result was inconsistent with carcinogenicity bioassay data. The different carcinogenicity of di(2-ethylhexyl)adipate (negative in rats) and di(2-ethylhexyl)phthalate (positive) was not detectable. Both plasticizers were positive in this short-term system with DD's of 0.7 mmol/kg for DEHA and 0.5 mmol/kg for DEHP. The proposed assay is discussed as an attempt to devise short-term assays for carcinogens not detected by the routine genotoxicity test systems.