Dose responses to two heterocyclic amines, 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA alpha C) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), in induction of glutathione S-transferase placental form positive liver cell foci (GST-P+ foci) and DNA adduct formation in the liver were examined in male F344 rats. Beginning 2 weeks after a single diethylnitrosamine (DEN) injection (200 mg/kg, i.p.), rats received MeA alpha C or PhIP in the diet at various doses for 6 weeks. All rats were subjected to two-thirds partial hepatectomy (PH) 1 weeks after the test agents were added to the diet and were killed 8 weeks after DEN initiation. MeA alpha C (100, 200, 400 and 800 p.p.m.) significantly increased numbers and areas of GST-P+ foci over control levels in all dose groups with a clear dose-response. In contrast, PhIP (50, 100, 200 and 400 p.p.m.) only equivocally increased foci development in the highest dose group and rather was associated with decrease in the lower dose groups. DNA adduct formation assessed by 32P-postlabeling demonstrated a dose-dependent increase with both chemicals, the levels being much higher with MeA alpha C. Thus, two highly mutagenic heterocyclic amines that are produced in broiled foodstuffs exerted different influence on GST-P+ foci development and DNA adduct formation; these findings are consistent with liver carcinogenicity in rats and/or mice.