AbstractRats were injected with 5 mg NaF/100 g or 125 mg SrCl2/100 g. Animals injected once were killed at intervals of 4, 6, 12 and 24 days after injection. Other animals received pairs of injections, separated by intervals of four or nine days. One maxillary incisor from each animal was embedded in methacrylate, sectioned transversely, and microradiographs prepared. The other incisor was demineralized, sectioned, and the sections stained with histochemical methods for polysac‐charide‐protein complexes.In the microradiographs, the abnormally mineralized layers of dentin produced by the injections became less distinct as they aged. This recovery was detectable in all animals within two weeks after injection. The histochemical changes were less marked, but the weakly stained (hypomineralized) layers of matrix became more nearly normal as recovery occurred. Apparently, fluoride accelerates the maturation (secondary mineralization) of a thin layer of dentin, while both fluoride and strontium inhibit primary mineralization of predentin. In time, mineralization in the hypomineralized layers overtakes the normally mineralized dentin, and the normally mineralized dentin overtakes the hypermineralized lay
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