This study assessed the incorporation of fluoride into remineralized enamel and the stability of the incorporated fluoride under various test conditions. Lesions were produced on bovine enamel slabs by a two-day immersion in 0.01 mol/L lactic acid buffer containing 3.0 mmol/L Ca, 1.8 mmol/L P, and 1% CMC adjusted to pH 4.0 at 37 degrees C. The remineralizaing solution contained the same amount of Ca, P, and CMC, plus 150 mmol/L NaCl and 3 ppm F, and was adjusted to pH 7.0 at 37 degrees C. All slabs were exposed to this unstirred solution, which was changed every two days during the ten-day remineralizing period. The remineralized slabs were divided into four groups. Group A (the control group) received no further treatment. The other three groups were exposed for 24 h to either the intra-oral environment (Group B), a 1.0 mol/L KOH solution (Group C), or a 0.01 mol/L lactic acid buffer (Group D). Fluoride incorporation assessed by abrasion biopsy in 10-microns layers showed about 10,000 ppm F maximum in Group A. Similar levels of fluoride concentration from the surface to approximately 30 microns thick were found in Groups A, B, and C. No appreciable fluoride was released from remineralized slabs from Groups B or C, and only a small fraction from Group D. Statistical analyses of the fluoride values showed no significant differences between the various test conditions in any of the layers sampled. No difference was evident in the Ca/P ratio between the ten-day remineralized enamel (Group A) and the treatment groups. The lack of appreciable fluoride loss from enamel with any of the above three conditions indicated a stable fixation of fluoride in the remineralized enamel lesions.