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The discovery of electric-field effect for conducting polymers in transistor structures aroused a number of questions about structure, mechanism of charge transport, and a role of ions in conducting polymers. We present here the model of an electrochemical transistor whose resistance is governed by the gate potential through bulk ionic charging/discharging of the conducting polymer-based active channel. The predicted I(V) characteristics do not agree with the measured experimental dependencies for highly doped conducting polymer-based transistors. We suggest that the observable electric-field effect in conducting polymers is related to their structural peculiarities. The large free volume within the conductive polymer chain network enables ions to easily move into and out of the polymers. The main effect of ion insertion is breaking of the percolation network for the conductivity by removing critical hoping sites and, as a result, producing a conductor-nonconductor transition.