An improvement of the description of the nonlinear behavior of PZT piezoelectric ceramics is proposed. The current obtained when applying a low frequency and high voltage harmonic signal is analyzed using a bridge, built ad hoc. The validity of no model is presupposed in the processing of data. Nonlinear behavior depends explicitly on the amplitude of the applied field. The nonlinear increment of the dielectric constant can be split into three fractions: the first one k f , is directly related to the losses, does not depend on the instantaneous field, and is larger in non aged ceramics. None of these effects occurs with the second fraction k g , related to reversible processes. The dependence of those terms on the field amplitude is of the power type, with fractional power that depends on the ceramic type.