Polarization fatigue in ferroelectric perovskite oxides is a commonly known phenomenon. However no model has promoted consensual interpretation regarding its nature. In this article, the polarization loop of a Pb(Zr0.54Ti0.46)O-3 thin film capacitor measured at a given moment of the fatigue process is entirely duplicated after bringing in series with a fresh sample an electrical circuit calibrated in a way to model the aging mechanism. The latter was analyzed by considering time dependent interface properties but invariant bulk values. Unequivocal demonstration is made that fatigue polarization in the studied sample resulted from decline of the electric properties of the interfaces, leading to a decrease in the electric field actually applied to the switching domains. Injection of charge at interface defects, caused by polarization reversal, was probably the vector of these degradations. (C) 2012 Elsevier B. V. All rights reserved.