To investigate the mechanism of lesion remmeralization, artificial (HEC) lesions in bovine enamel were remineralized in a pH and pF-stat controlled system at 37 °C. In all experiments 5 cm2 lesion was immersed in 12.5 ml of a solution containing 2 mM Ca, 1.2 mM PO4 and either·or 1 ppm F at pH = 7.0. The mineral deposition was followed by monitoring the alkali and fluoride uptake and the solution calcium and phosphate concentration changes. From the ratio of the deposited ions (Ca/PO4, Ca/F, Ca/OH, etc.) and from the analysis of the lesion material after remineralization (IR data, X-ray diffraction), the conclusion can be drawn that the material deposited is most likely (fluoridated) hydroxyapatite (HAP). SEM experiments show that a crystalline material is deposited during remineralization, with rod-like crystals having a diameter of 200 nm. The addition of 1 ppm F to the remineralizing solution causes an about twofold increase in rate of remineralization thus comparable with previous studies of the remineralization of acid-etched enamel. Hardness experiments indicate that during the deposition studied a rehardening of the body of the lesion occurs.