The percolation threshold potential was defined as the potential at which the conducting zone began to grow in a dopant-rich solution when the conducting polymer was electrochemically switched from the insulating to the conducting state. This potential is an electrochemical measure of the percolation threshold of the molar fraction of the conducting species. It was evaluated from the dependence of the growth rate of the conducting zone in the polyaniline film, which was observed through a video camera, on the electrode potential E. The conducting zone developed at a constant rate whose logarithm was linear with E at potentials more positive than 0.3 V/SCE. At potentials less than 0.3 V, the rate decreased rapidly with decreasing E and vanished at E=0.22 V. Therefore the percolation threshold potential was 0.22 V. Chronoamperometry was used at E < 0.22 V where no growth was observed visually. The response current at E < 0.10 V was inversely proportional to the square root of the electrolysis time, and hence the diffusion-controlled current of the reduced or insulating species in the film. The current for 0.1 V < E < 0.20 V was inversely proportional to time, like the slow relaxation observed in the reduction of conducting polymers. The current E > 0.24 V showed a plateau, suggesting the transient predicted from the propagation theory. The shape of the current-time curve altered at potentials ranging from 0.20 to 0.24 V. The threshold potential not only has thermodynamic implications but is also more practical than the standard or formal potential in electrochemical switching.