A simple model of polymeric relaxation, associated with the electrochemical switching of polypyrrole films between their reduced (insulating) and oxidized (electronically conducting) states, offers a reasonably precise description of the form of chronoamperograms obtained after previous subjection of the polymer film to cathodic potentials (which control the compactness of the neutral polymer) for long periods of time. The opening of the structure driven by the anodic potential is not uniform : nucleation of conducting zones inside the neutral polymer and their overlap at long times of anodic polarization are taken into account in the model. Diffusion of counterions from the solution across the oxidized zones is also included. The definition of a relaxation time (depending on both cathodic and anodic overpotentials and on temperature), and the inclusion of nucleation and diffusion processes, allows a theoretical simulation of chronoamperograms, in good agreement with experimental results from potential steps performed by changing the anodic and cathodic limits or at different temperatures.