One hundred individuals were given 25 ml of a 1:1000 aqueous solution of NaF to hold in the mouth for 2 or 3 mins. On the basis of increase in vol., it was concluded that the fluoride was diluted by an avg. of 3.2% in 2 mins. and by an avg. of 5.2% in 3 mins. Ten other individuals expectorated less than the 25 ml, and may have swallowed 6% of the fluoride soln. The toxicity of swallowed fluoride should be considered in prescribing fluoride mouth rinses.