Naturally occurring coumarins (NOCs) are anti-carcinogenic in the mouse skin model. To characterize the chemopreventive potential of NOCs against breast cancer, we first examined their effects on 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adduct formation in mouse mammary gland. We hypothesized that those NOCs that both inhibited cytochrome P450 1A1/1B1 and induced hepatic glutathione S-transferases (GSTs) would be the most effective in blocking DMBA-DNA adduct formation in mouse mammary gland. To address this hypothesis, simple coumarins (e.g. coumarin and limettin, which induced mouse hepatic GSTs but had little effect on P4501A1/1B1) and linear furanocoumarins (e.g. imperatorin and isopimpinellin, which induced hepatic GSTs and were potent inhibitors of P4501A1/1B1) were compared. Mice were pretreated with NOCs (150 mg/kg body wt, by gavage) prior to either a single dose of DMBA (50 microg) or multiple doses of DMBA (20 microg daily for 3 and 6 weeks). Mammary DMBA-DNA adduct formation was quantitated by the nuclease P1-enhanced 32P-postlabeling assay. With the single dose of DMBA, coumarin, limettin, imperatorin and isopimpinellin inhibited DMBA-DNA adduct formation by 50, 41, 79 and 88%, respectively. Coumarin, limettin and imperatorin blocked DMBA-DNA adduct formation by 36, 60, and 66% at 3 weeks, and by 0, 49 and 55% at 6 weeks of DMBA dosing, respectively. In a 6 week dose-response study of select NOCs and 7,8-benzoflavone (a potent P4501 inhibitor that had little effect on GSTs), DMBA-DNA adduct formation was inhibited by 0, 43 and 24% in the limettin groups; by 26, 26 and 69% in the isopimpinellin groups; and by 80, 96 and 97% in the 7,8- benzoflavone groups at 35, 70 and 150 mg/kg, respectively. Taken together, these results suggest that linear furanocoumarins had a greater inhibitory effect on DMBA-DNA adduct formation in mouse mammary glands compared with simple coumarins, and that the predominant effect may be P4501 inhibition.