Nitrite-induced transformation of 3-amino-4-monomethylamino-2',7'-difluorofluorecein (DAF-FM) to the triazol form (DAF-FMT) was studied using bacterial fraction of mixed whole saliva. The transformation at pH 7 was inhibited by SCN(-), suggesting that nitrosative stresses were small in the oral cavity at the pH value as SCN(-) was a normal component of saliva. DAF-FMT formation was much faster at pH 5.2 than 7 and ONSCN generated by the reaction of HNO(2) with SCN(-) mainly contributed to its formation at pH 5.2. Coffee and phenolic antioxidants inhibited the DAF-FMT formation less effectively at pH 5.2 than pH 7. The less effectiveness was discussed to be due to slow scavenging of ONSCN, which contributed mainly to the formation of DAF-FMT at pH 5.2, by coffee and phenolics. Since nitrite-induced formation of ONSCN should become faster as the pH decreases, it was suggested that nitrosative stresses to oral tissues became serious under acidic conditions.