The root canal length is measured during the endodontic treatment in order to determine the working length i.e. the length of the root canal instrumentation. When the dental pulp is infected, it should be completely removed from the root canal. If inflamed tissue remains in the canal or the cleaning is carried too deep damaging the surrounding tissue, unnecessary complications may occur. Therefore, the success of whole endodontic treatment is directly dependent on the accuracy of determining the position of the apex. The root canal length may be measured using radiographs. However, the accuracy of such measurements is limited because the projection of the curved and overlapped canals on the film does not always indicate the actual canal length. Therefore, after preliminary radiographs, an electronic apex locator or endometer is commonly used to determine the working length. They work on the principle of electrical impedance measurement between the active electrode inside the root canal (an endodontic file) and a neutral electrode placed at the oral mucosa. The first endometers have used direct current to measure the resistance between the electrodes, which proved to be unreliable because of the polarization potential at the metal to electrode interface. Therefore, present devices instead of measuring resistance measure the impedance at one or more frequencies. Based on that measurement the position of endodontic instrument inside the root canal is determined and indicated. It was found that the presence of conductive fluids that are commonly used during the endodontic treatment may influence the apex locator accuracy as well as the other parameters like the root canal width, morphology of the root, the possible lateral canals or perforation. The problem with verification of radiographic and electronic methods of determining the root canal length in vivo is that the actual position of the active electrode in the canal can be verified (by optical microscope) only when the tooth is extracted. Therefore, comparison of different electronic apex locators is usually performed in vitro on a physical model or by comparing results with the radiograph. The in vitro physical model usually consists of an extracted and prepared tooth immersed in to a conductive medium. Several of these in vitro models exist, and they are used arbitrary and non-critically. Therefore their electrical behavior is evaluated and compared in this study. There are plenty of papers that concentrate at endometer accuracy verification, however there are only a few that focus on the electrical properties of root canal. In these papers the total impedance in the measurement circuit is presented by a simple electrical equivalent circuit containing of two or three elements like resistors and capacitors. It is shown that these circuits can describe the impedance in narrow frequency band only. In order to achieve reasonable agreement in a wider frequency range, the elements needs to be frequency dependent, making them useless for root canal length determination purposes. Therefore, more complex equivalent circuits with up to five elements including a constant phase element is developed. When the number of free circuit elements rises, especially if they are a constant phase elements, the complexity of parameter estimation rises as well. Circuit element estimates became dependant not only on desired impedance spectrum, but on each other as well. Local convergence minima in the iterative estimation procedure occur. In this thesis the parameter estimation procedure for given equivalent circuits is defined and the circuits are compared according to the fit quality and parameter utilization efficiency. Physical representation of individual equivalent circuit elements are discussed based on witch a new measurement procedure is proposed.