In a previous investigation of fluoride retention after mouth-washing a group of adults retained more fluoride than children in spite of more complete expectoration of the children. It therefore appeared possible that certain oral structures, such as calculus deposits and silicate fillings as well as intact enamel might have taken up important quantities of fluoride. In the present study an attempt was made to assess the magnitude of the storage in the tooth surfaces and to determine to what extent dental calculus and silicate fillings moderate this uptake. By the aid of 18F the uptake was studied in vitro in surfaces of intact enamel, calculus deposits and silicate fillings exposed to sodium fluoride solutions and in vivo after mouth-rinses in the following groups of patients: 1. patients with all teeth missing, 2. patients where single intact teeth with calculus deposits were to be extracted, and 3. patients where single teeth were to be extracted. Both in vitro and in vivo studies showed that the uptake in the tooth-crowns is responsible for only a small fraction of the total fluoride loss. This uptake is 5.5 and 8.5 times higher in silicate fillings and dental calculus, respectively, than in intact enamel. The fluoride uptake of the total intact enamel surfaces after mouth-washing can be calculated to only about 1% of the total fluoride retention. Retention probably takes place by direct absorption in the oral mucosa and by involuntary swallowing.