Studies have been undertaken to determine the uptake and release of fluoride from hydroxyapatite and it has been concluded that the effect of a fluctuating pH on hydroxyapatite in the presence of a fluoride solution is to increase the fluoride content of the hydroxyapatite by a mechanism involving: (1) partial dissolution of hydroxyapatite at pH 4.0; (2) initial formation of calcium fluoride at pH 4.0 and (3) transformation of calcium fluoride into fluoroapatite as the pH increases. An attempt has been made to correlate these findings with other reports describing the incorporation of fluoride into mineralised hard tissues.