A new apparatus for the de- and remineralization of enamel in vitro under constant composition conditions is described. A de- or remineralizing solution flows over the specimens at a controlled rate. During artificial lesion formation (using an acetate buffer containing methylhydroxydiphosphonate, MHDP), the composition of the solution is constant. The Ca and phosphate variation is less than 3%, pH variation is less than 0.03 pH unit. For enamel these variations are within the biological variations between enamel specimens. Experimentally it was found that the liquid streaming over the enamel surface does not measurably remove material from the surface (erosion) with the experimental error of 0.1 μm. This was measured by following microhardness indentations during demineralization. The effect of various MHDP concentrations on demineralization was investigated in the range of 2–50 μM. Even low concentrations of MHDP prove to be effective in the microhardness increase and lesion depth reduction during demineralization. In the demineralization experiment it is shown by means of microradiography, hardness measurements as well as light microscopy that the method described quickly produces reproducible lesions. In a remineralization experiment of demineralized enamel effective remineralization of the lesions occurs in 360 h, giving a threefold increase in microhardness and a twofold decrease in lesion depth.