The synergism of two carcinogenic aromatic amines with different tissue specificities was studied at the level of initiation in Wistar rats. Gamma-glutamyl transpeptidase and glutathione S-transferase P were used as markers for preneoplastic foci in liver. 2-Acetylaminofluorene (AAF) is a complete rat liver carcinogen, whereas trans-4-acetylaminostilbene (AAS) produces ear duct tumors quite selectively, but also acts as a strong initiator in rat liver. When these carcinogens were administered sequentially as two doses of each or simultaneously as four doses of a mixture to neonate animals, which then were treated with phenobarbital in the drinking water for promotion, the initiating activity was additive. When these chemicals were given to young adult animals within 4 weeks in two series of four doses, followed by partial hepatectomy and phenobarbital in the drinking water, the number of preneoplastic foci was greater in groups which had received AAS in both series or in the second series after AAF than in those groups which had received only AAF or AAF in the second series. The average size of foci depended clearly on the sequence in which the two carcinogens were administered. The foci were larger when AAF was given after AAS. The results support the notion that AAS is a strong initiator in rat liver, and that AAF, which is a complete liver carcinogen, has promoting properties under certain circumstances in addition to its initiating properties. The two carcinogens seem to produce the initiating lesions independently but the extent of initiation is additive in this model situation. The simplified neonatal rat liver model appears to be particularly suitable for investigating initiating properties and is proposed for studies of synergistic effects of genotoxic chemicals on the initiation stage, independent of organotropism. It avoids a number of complicating factors related to treatment schedule, forced proliferation rate and toxicity in other models.