To explore for practical thresholds for DNA-reactive carcinogens in rat liver carcinogenicity, we have conducted a series of exposure-response studies using 2 well-studied hepatocarcinogens, 2-acetylaminofluorene (AAF) and diethylnitrosamine (DEN). Findings with AAF, including as yet unpublished experiments, are reviewed here and related to DEN observations. In these studies, we have administered exact intragastric doses during an initiation segment (IS) of 12-16 weeks followed in some experiments by phenobarbital (PB) as a liver tumor promoter for 24 weeks to enhance manifestation of initiation. The cumulative doses (CD) of AAF at the end of ISs ranged from 0.094 to 282.2 mg/kg. Our findings for AAF in the IS can be summarized as follows: (1) the earliest parameter to be affected with administration of low doses was the appearance of DNA adducts (around 4 weeks), followed at higher doses by cell proliferation; (2) formation of DNA adducts was nonlinear, with a no-observed effect level (NOEL) at a CD of 0.094 mg/kg and a plateau at higher doses (94.1 mg/kg); (3) cytotoxicity (necrosis) showed a NOEL at a CD of 28.2 mg/kg; (4) compensatory hepatocellular proliferation showed a NOEL at a CD of 28.2 mg/kg and was supralinear at a high CD (282.2 mg/kg); (5) formation of preneoplastic hepatocellular altered foci (HAF) showed a NOEL at a CD of 28.2 mg/kg, and was supralinear at a high CD (282.2 mg/kg); (6) a NOEL (CD 28.2 mg/kg) was found for tumor development and the exposure-response was supralinear. We interpret these findings to reflect practical thresholds for hepatocellular initiating effects of AAF and exaggerated responses at high-exposures doses, as also found for DEN. Thus, mechanisms of carcinogenesis can differ between low and high doses.