Demineralization of dentin in the presence of fluoride produces lesions with a mineralized surface layer which becomes thicker and more mineralized with higher fluoride concentrations whereas the lesion depth is hardly affected. The aim of this study was to investigate the effects of the time of fluoride treatment and the amount of fluoride taken up on the properties of the mineralized layer. Discs of bovine dentin embedded in methylmethacrylate with one surface exposed were demineralized in 50 mM acetic acid, 2.2 mM CaCl2, 2.2 mM KH2PO4, pH 5.0. At the start and/or later during the demineralization period, the specimens were incubated individually for 1 or 2 days in 10 ml of the same demineralization solution supplemented with 0.5, 2.0 or 5.0 ppm fluoride, which was then assessed for changes in calcium and fluoride concentrations. After 2, 5 and 8 days, specimens were sectioned for microradiographic analysis so as to follow development of the lesions and the mineralized layers. The results were the following: While demineralization with fluoride present at the first day led to the formation of a surface layer, fluoride present only at a later day produced a subsurface layer, not at the lesion front but closer to the surface. This layer resulted from (re)precipitation and not from preservation of the original mineral. The 'integrated mineral content' of the surface layer increased linearly with the uptake of fluoride, which resulted in an apparent fluorapatite content of about 20 vol%. The profiles of the surface layers remained unchanged during continued demineralization in the absence of fluoride. It was concluded that in the presence of fluoride mineral loss is reduced as a result of the reprecipitation of dissolved mineral ions as a layer of fluoride-enriched apatite. This layer does not offer protection of underlying dentin against continued demineralization.