This investigation examined the complicated impedance characteristics of the human tooth root canal using a mimic system that comprised a cylindrical tubule and 5% agar-gel. The in vitro impedance measurements of root canals were made using a frequency response analyzer and a tubule filled with distilled water or normal saline. An equivalent circuit model of this bioelectric system was proposed; it closely corresponds to the experimental results. On the basis of the proposed equivalent circuit model, the experimental results can be explained reasonably and the effect of the filling of the canal with the electrolytes can be clearly understood. The modified impedance quotient (Q) and the variation ratio of the impedance quotient (S-Q) are taken as the useful indices of the apex. Different impedance behaviors have been identified from Q values in under-apex and over-apex regions, respectively. The S-Q was < 0.15 at the apex location as well as in the overapex region for all electrolytes and could be used to eliminate the effect of the electrolytic conductivity. This study elucidates the complicated electric mechanisms of electrolytic systems and improves clinical applications in the endodontic treatment. (C) 2008 The Electrochemical Society.