The covalent binding of the ultimate carcinogen (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [alpha]pyrene (BPDE) to enriched ovalbumin messenger RNA (mRNAov) of known sequence was examined. Incubation of mRNAov with elevated concentrations of labeled BPDE in TE buffer (0.02 M Tris X HCl, 1 mM EDTA, pH 7.2) containing 0.1 M KCl and 10 mM MgCl2 resulted in approximately 30 BPDEs covalently bound per RNA molecule. Covalent binding in the absence of KCl and MgCl2 resulted in a significant increase in binding to 110 BPDEs bound per molecule or modification of 12% of the total guanosine and adenosine nucleotides present. The nucleoside adducts formed were nearly all guanosine and adenosine in a ratio of 1.6:1.0. It was also observed that digestion of mRNAov with T2 RNase prior to reaction with BPDE resulted in a 52% decrease in guanosine adduct formation and a 93% decrease in adenosine adducts compared with undigested controls. Comparison of the binding of labeled BPDE to 18 S and 28 S ribosomal RNAs and to mRNAov revealed that the guanosine adduct to adenosine adduct ratio and the number of BPDEs bound increased with increasing G-C content. The results reported here show that ionic composition of the medium, G-C content, and the presence of a polymeric state can significantly influence the quantitative and/or qualitative nucleoside BPDE adducts formed.