One of the peaks present in HPLC profiles of [3H]benzo[a]-pyrene (BaP)-deoxyribonucleosides prepared by enzymatic degradation of [3H]BaP-DNA isolated from Wistar rat embryo cell cultures exposed to [G-3H]BaP was found to be r-7,c-9,c-10,t-8-tetrahydroxy-7,8,9,10-tetrahydroBaP, a BaP-DNA adduct decomposition product (Pruess-Schwartz, D. and Baird, W.M., Cancer Res., 46, 545-552, 1986). To investigate the stability of the hydrocarbon-deoxyribonucleoside linkages in intact BaP-modified DNA, DNA was isolated from Wistar rat embryo cells that had been exposed to [G-3H]BaP and incubated in darkness at 37 degrees C at a range of pH values from 5 to 11 for 72 h or for 1-150 h at pH 7. The rate of breakdown of [3H]BaP-DNA adducts (0.25%/h) was linear over 150 h. The amounts of the two major BaP-DNA adduct decomposition products, I and II (present in a ratio of 1:3), increased with length of time of incubation. Formation of I was not affected by pH, whereas, formation of II was highest at acidic and neutral pH. Analysis of the decomposition products by immobilized boronate chromatography and reverse-phase HPLC demonstrated that both I and II contained cis-vicinal hydroxyl groups and decomposition product II cochromatographed with r-7,c-9, c-10,t-8-tetrahydroxy-7,8,9,10-tetrahydroBaP, a (+/-)-7 beta,8 alpha-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydroBaP (syn-BaPDE)-derived tetraol. At neutral pH [3H](+/-)-syn-BaPDE-modified calf thymus DNA formed a decomposition product identical to II. Analysis of the BaP-DNA adducts that remained covalently bound to the DNA after the above incubations demonstrated that the amounts of both major syn-BaPDE-deoxyguanosine adducts decreased with length of time of incubation. Thus, syn-BaPDE-deoxyribonucleoside adducts formed in the DNA of [3H]BaP-treated Wistar rat embryo cells are unstable and breakdown spontaneously in the absence of light to yield syn-BaPDE-tetraol decomposition products.