Both trans-4-acetylaminostilbene (AAS) and 2-acetylaminofluorene (AAF) exert tumor-initiating activity in rat liver when administered in the initiation phase of an initiation-promotion experiment. The effects are more than additive when the compounds are sequentially combined in the initiation phase of such an experiment, and this synergism is more pronounced when AAS is given first, followed by AAF, than vice versa. In order to determine the role of target DNA dose, [3H]AAS and [14C]AAF were administered to female Wistar rats adhering to the protocol of the initiation phase of the above-mentioned experiment and the following parameters measured at the end of this phase: total radioactivity in tissues, binding to DNA, RNA and proteins in liver, adduct pattern in liver DNA and RNA. In neither combination were these parameters significantly different from those in the appropriate controls in which only one of the compounds was administered. This result indicates that combining the substances did not alter the pharmacokinetics of the individual compounds and that the target dose is additive. This suggests that effects unrelated to DNA binding, possibly promoting effects, may cause the more than additive generation of preneoplastic lesions in rat liver.