Formaldehyde reacted with guanosine and the products were assayed using fluorescence spectroscopy at different pHs. The reaction products were fluorescent in alkali only as was found with commercial N-2 methylguanosine. Thus, formaldehyde appeared to react with N-2 of guanosine. The reaction was completed with 30s both at 0 degrees C or at 21 degrees C. The stability of the reaction products was further investigated by the addition of glycine. When formaldehyde was allowed to react with guanosine for 30s, 90% of the fluorescence disappeared when glycine was added suggesting a labile adduct such as a methylol derivative. By contrast, after a 20 h incubation only 20% of fluorescence was abolished with glycine suggesting a stable adduct such as a methylene derivative.