It has been reported that the fluoride released from an intraoral fluoride releasing device (IFRD) penetrates into the enamel, strengthening both the surface and subsurface layers. The goal of this study was to investigate the acid solubility of both surface and subsurface enamel treated with the IFRD. In order to experiment with stable conditions, rather than use the IFRD, the enamel samples were immersed in three different concentrations of fluoride solution, which could have been maintained by IFRD. The concentrations of fluoride solution were 10.0 ppm, 1.0 ppm and 0.3 ppm. Enamel slabs were prepared from bovine permanent incisors and immersed in the fluoride solutions for 30 to 90 days. After immersion, enamel slabs were demineralized in acetate buller at pH 1.1 for 48 hours. The amount of dissolved calcium was determined in order to evaluate the reduction of enamel solubility. After demineralization, the enamel which had been immersed in the fluoride solution for 90 days was observed by SEM. The results obtained were as follows: 1. The fluoride uptake and penetration in the enamel increased with time and with an increase in the fluoride concentration. 2. According to the results of the dissolution test, acid solubility decreased in proportion to the increase in fluoride level in both the surface and subsurface enamel. 3. The immersion time in the fluoride solution did not affect the acid solubility when the fluoride concentration was low. However, for higher concentrations, an increase in immersion time directly decreased acid solubility. 4. There was considerable cavitation in the control group both in the surface and subsurface enamel. The enamel surface of the fluoride groups for the most part showed much less cavitation. Cavitation in the subsurface enamel of the 0.3 ppm group was as great as that with the control group. The greater the increase in the concentration of the fluoride solution, the smaller the cavitation in the subsurface enamel.