The observations of interest made in this investigation may be summarized as follows: 1. Conventional fluoride application by mouthrinsing with neutral NaF solution (0.025% daily for three weeks or three months; or 0.05% solution daily; or 0.2% solution weekly for three weeks) is found to result in only moderate and insignificant uptake of F in facial enamel surfaces. The gain is in the order of a hundred ppm within 10 to 15 microns' depth (as determined by an acid etching technique) and a few hundred ppm at 0.02 to 0.05 microns (according to pilot ion probe analysis). 2. Painting, at one week's interval for three weeks with 2% NaF (pH=5) solution and application of an APF-gel (Flura-Kerr 1.23% F) results in the significant uptake of a few hundred ppm F within the first 10 microns of the enamel. Painting with 2% NaF (pH=7), with 8% SnF2, or with 5% Na2PO3F solutions does not yield a significant gain in F in the first 10 microns of the facial enamel, as determined by the etching technique under similar conditions. 3. A varnish (Duraphat) containing 5% NaF (applied three times at weekly intervals) is found to deposit considerable amounts of fluorine in outer facial enamel. Within the first 10 microns gains of more than 1000 ppm F are detected by the etching technique. Uptakes of the order about 500 ppm F are seen even at depths beyond 10 microns; penetration of F is detected down to about 100 microns' depth. Ion probe experiments show a F gain of more than 2000 ppm at a depth of about 0.3 microns. 4. In teeth extracted one day after a single application of NaF varnish, the retained fluorine is higher than in teeth retained in oral environment for one or five weeks after application. This suggests that loss of F occurs particularly during the first days after treatment. 5. The varnish, when applied every six months in permanent dentition of 15 year old children, is cariostatic on all tooth surfaces. 6. The secondary ion microanalyzer ("ion probe") is a useful instrument for the study of fluorine concentrations in dental hard tissue, and provides a special approach to the outermost regions of enamel. 7. The ion probe renders quantitative F determinations in dental hard tissue; a calibration method is available when the positive secondary ion ratios CaF+/Ca+ or F+/Ca++ are recorded versus Ca++/Ca+.