Formulating an effective toothpaste formula requires that fluoride (F) in toothpaste must be made available to the enamel microenvironment in reactive form (bioactive). The aims of this study were to evaluate the compatibility of F with the abrasives (cleaning and polishing agents) used in toothpaste formulations and to monitor the effect of ageing on availability of F in NaF/abrasive mixtures as well as in toothpaste formulations. There was a great variation in the rate and extent of loss of F in the NaF/abrasive mixtures and in the toothpaste formulas. Addition of NaF solution to aluminium- and calcium-containing abrasives resulted in losses of 60-90 per cent of the added F after 1 week's storage at room temperature. At 1 month of age, calcium phosphate dihydrate and aluminium silicate adsorb about four times more F than calcium carbonate. On the other hand, sodium bicarbonate and sodium metaphosphate inactivated 20-25 per cent of the added F after 9 months' storage. Silica was inert in binding and inactivating F of NaF and Na2PO3F (MFP). Calcium-containing abrasives were markedly more compatible with MFP than with NaF. After 12 months of storing the toothpastes, the soluble F (initial F- plus F of hydrolysed PO3F2-) in MFP-calcium carbonate/sodium silicate toothpaste was twice as much as MFP-calcium carbonate/aluminium silicate toothpaste.(ABSTRACT TRUNCATED AT 250 WORDS)