The stimulation of unscheduled DNA synthesis (UDS) by 3-amino-1,4-dimethyl -5H-pyrido[4,3-b]indole (Trp-P-1), 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2), 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) and 2-amino-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2) was measured in primary cultures of rat hepatocytes. Hepatocellular DNA was recovered as a cetyltrimethylammonium salt from nuclei digested with protease. The incorporation of [3H]thymidine was measured by liquid scintillation counting techniques. Although Trp-P-1 was the most potent of the pyrolysis products tested, it was considerably less active than 2-aminofluorene (2-AF) or aflatoxin B1 (AFB1). A greater induction of UDS by Trp-P-1 was observed in cultures from female rats than in cultures from male rats. Aroclor 1254 pretreatment resulted in a substantially greater UDS response to all 4 pyrolysis products.