There are increasingly reports on sensitivity to pressure on teeth filled with composite resins in the days following the filling, a fact that has induced the Authors to carry out a research "in vitro" and "in vivo" in order to study variations of the electrical potential of teeth filled with composite materials. The study has shown that this potential varies considerably in intensity according to the product used for tests in vitro and in vivo, probably due to polimerization defects of the material. These are the first results of a study which will be further extended and which highlights new and so far unknown aspects of widely used filling materials.