The interaction of guanosine with 7,12-dimethylbenz[a]anthracene (DMBA) 5,6-oxide under alkaline conditions resulted in the formation of six derivatives. These six compounds were cochromatographed with nucleosides obtained by hydrolysis of RNA isolated from rat liver cells treated with [3H]DMBA. The cochromatography showed that three of these adducts were formed in cellular RNA. The three products constituted less than 5% of the total nucleoside-DMBA adducts as shown by chromatography on Sephadex LH-20 and high-pressure liquid chromatography. In one of them, the 2'-hydroxy group of the ribose moiety of guanosine was linked to the C-5, and in the second, to the C-6 position of the DMBA 5,6-oxide residue. In the third derivative, the C-8 position of guanosine was linked to the C-5 of the DMBA 5,6-oxide moiety. These results show, for the first time, modifications of the ribose moiety and of the guanine residue at the C-8 position in the cellular RNA by a metabolite of a polycyclic hydrocarbon.