The fluoride ion activities of two NaF dentrifrices have been determined in undiluted samples, in samples diluted in 6 ml water or saliva, and in saliva-dentifrice slurries after 2 min brushing. The activity potentials were compared with the activities of NaF standards. The fluoride ion concentration of each dentifrice was determined when 0.1 g dentifrice was dissolved in 5 ml water and 20 ml buffer. The activities in diluted samples were about twice the activities in in vivo samples. The fluoride ion activity in saliva before fluoride application was below 0.06 ppm and rose to about 1 ppm half an hour after fluoride application.