The proportions of individual benzo[a]pyrene (BaP)-DNA adducts present in rodent embryo cell cultures change with the length of time of exposure to BaP; the major alteration is an increase in the proportion of (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydroBaP (BaPDE)-deoxyguanosine (dG) adduct (Sebti et al., Cancer Res., 45 (1984) 1594-1600). To determine if this change in the BaP-DNA adducts could result from the induction of enzymes involved in oxidation of BaP, hamster embryo cell cultures were exposed to acetone or BaP for 24 h and then the medium was replaced with fresh medium containing [3H]BaP. After 5 h the BaP-pretreated cells had a 30% higher level of binding of BaP to DNA and formed a greater proportion of (+)-anti-BaPE-dG adduct than the acetone-pretreated control group. Cells pretreated for 24 h with BaP and then exposed to [3H]BaP and Actinomycin D for 5 h had a lower level of binding of BaP to DNA and a lower amount of (+)-anti-BaPDE-deoxyguanosine adduct than cells pretreated with acetone and exposed to [3H]BaP for 5 h. In contrast, pretreatment for 24 h with BaP plus Actinomycin D followed by a 5-h exposure to [3H]BaP resulted in a decrease in overall binding of BaP to DNA but had no effect on the amount of (+)-anti-BaPDE-deoxyguanosine adduct. Actinomycin D treatment had no significant effect on either the total amount of BaP metabolized, the formation of primary and water-soluble BaP metabolites, or cell viability, but reduced [3H]uridine incorporation into RNA by more than 65% at all times. These results suggest that induction of specific isozymes of cytochrome P-450 may be involved in the time-dependent increase in the proportion of (+)-anti-BaPDE-DNA adducts in BaP-treated cells. The state of induction of specific isozymes of cytochrome P-450 and the ability of the BaP dose applied to induce them may be major factors in determining the proportion of BaP metabolized to (+)-anti-BaPDE, the most carcinogenic stereoisomer of BaPDE.