In this in vivo study the amount of fluoride present on and in human enamel was followed longitudinally for a period of 3 months. The fluoridating agents were an APF gel and the fluoridating lacquers Duraphat and Fluor Protector. The fluoride on the enamel (Fon, mainly as CaF2) was determined by the Caslavska method. The fluoride in the enamel (F2) was measured in five enamel layers removed by acid etching. 12 patients wore the treated enamel specimens and controls for periods of 1, 4 and 12 weeks before the Fon and F2 determinations were made. The CaF2 (or Fon) was lost in vivo in all three treatments at a rate of about 20 μg cm-2 in the first week. Consequently, APF gel- and Duraphat-treated specimens lost nearly all CaF2 in vivo in this period. The amount of CaF2 present in the Fluor Protector case was noticeable up to 1 month. A second conclusion from this study is that the amount of acquired F- (F2 in a layer of 30 μm) following APF gel or Duraphat treatment was negligible after 1 week in vivo. A Fluor Protector treatment introduced an amount of acquired fluoride of 11, 14 and 15 μg cm-2 after 1 4 or 12 weeks, respectively. This study shows that if sufficient CaF2 is deposited on the enamel in vivo for a sufficiently long time, the amount of F∼ in the enamel can be increased significantly. Thirdly, despite the fact that during the in vivo experiments considerable amounts of CaF2 were lost in the saliva, no measurable enrichment of control sound enamel resulted, not even in sound enamel specimens 3 mm from the treated blocks.