Misonidazole [a radiation sensitizer which has been used in tumor radiation therapy], after reduction to the hydroxylamine derivative, reacted with guanosine in aqueous solution at pH 7. The guanosine product was isolated and was assigned a structure having a new 5-membered ring with a .sbd.CHOH-CHOH-linkage between the N-1 and N-2 positions of guanine. Removal of the sugar residue from the guanosine product by acid hydrolysis resulted in the corresponding guanine derivative, which was also made by reacting guanine with reduced misonidazole. In aqueous solution at pH 11, the guanine product was quantitatively converted to guanine within 20 min. A number of N-1-substituted 2-nitroimidazoles and 2-nitroimidazole reacted with guanosine in an analogous manner, giving rise to the same product as misonidazole, indicating that the C-4-C-5 fragment from the imidazoles is involved in the modification. Neither misonidazole nor its amine or hydrazo derivatives reacted with guanosine. Reduced misondazole reacted with N-2-methyl guanosine; with N-1-methyl guanosine, a reaction was not detected. Reactions such as the modification of guanine provide a possible molecular mechanism for the cytotoxic and neurotoxic properties of misonidzole.