Topically applied fluorides have only been partially effective in reducing caries. This study will investigate the effect of the pretreatment of enamel with polyvalent metal ions (iron (ferric), titanium and zirconium) as a means of increasing the efficacy of topical fluoride treatments. Teeth will be sectioned longitudinally into halves, one half will act as the test (pretreated) and the other as the control (APF treated). The experimental series will be divided into two phases: Phase A: The fluoride uptake from APF after pretreatment will be compared with the fluoride uptake from APF alone. In addition, the effect of this fluoride uptake on in vitro caries-like lesion formation will be determined. Caries-like lesions, which are histologically indistinguishable from naturally occurring ones, will be produced using an acidified gel technique. The lesions from two adjacent sections, one test and one control, will be compared by carrying out inhibition studies in polarized light. Phase B: Attempts will be made to identify the products formed on and/or in surface enamel after the pretreatment regimen. This will help to establish the mechanisms by whicha) polyvalent metal ions produce an elevated fluoride level in enamel, b) the pretreatments retard in vitro caries-like lesion formation. The techniques to be used in this phase of the work include polarised light microscopy, scanning electron microscopy and microprobe analysis. In this way, the changes in concentration and distribution of ions in enamel during lesion formation can be determined. Also, metal ions used as pretreatments (which inhibit lesion formation) will be combined with fluoride (e.g., TiF4) in a single solution. The effectiveness of this solution (as assessed in the dual experimental approach outlined above) will be compared with the corresponding metal ion pretreatment.