Sodium fluoride (NaF) and sodium monofluorophosphate (MFP) are drugs used to increase bone mass. They have been considered equivalent but the results of the treatments were not always coincident. Most studies have been carried out in osteoporotic women or ovariectomized rats pointing to the result in bone mass rather than at the mechanism of action. Convincing evidence indicates that pharmacokinetic of NaF is different from MFP. While only fluoride is found in bones and plasma of rats treated with NaF, in MFP-treated rats, there are also fluorine (F) bound to plasma alpha-macroglobulin and bone covalently bound F. A significant increase in bone mass of rats was observed after 30 days of treatment with NaF and MFP in young rats. This increase in bone mass correlates with the increase in number and thickness of trabeculas in cancellous bone. In the femur of MFP-treated rats, there was an increase in the inertia momentum of the diaphysis without changes in bone width. In addition, bone F content of MFP-treated animals is twice of the content of NaF-treated rats. This difference is the consequence of bone covalently bound F, which is absent in NaF-treated rats. In addition, alpha-macroglobulin was detected in noncollagenous bone matrix of MFP-treated rats. Although F in feces and plasma did not differ among treatments, the urinary excretion of F was lower in MFP than in NaF-treated rats, which is consistent with the higher bone F content.