Freshly isolated mammary epithelial cell aggregates from female Sprague-Dawley rats metabolized 7,12-dimethylbenz-[a]anthracene (DMBA) to bay-region anti- and syn-dihydrodiolepoxides that bound to deoxyguanosine and deoxyadenosine residues in cellular DNA. After 24 h of incubation 68% of the DMBA (0.4 micrograms/ml) was metabolized and 58% of the extracellular metabolites were water-soluble. DMBA-DNA binding increased rapidly during the initial 24 h of incubation. Formation of the bay-region syn-dihydrodiolepoxide:deoxyadenosine adduct increased linearly throughout the 24 h, whereas formation of deoxyadenosine and deoxyguanosine adducts with the bay-region anti-dihydrodiolepoxide increased rapidly following a delay of 12 h.