The ac field dependence of dielectric response of sol-gel derived Pb(Zr, Ti)O-3 thin films is presented. The analysis of amplitude and phase angle of first and third harmonic of the polarization at subswitching fields shows that a description of the dielectric nonlinearity by the classical polynomial approximation is inadequate. A qualitatively better description of many features of the experimental data is obtained using the Rayleigh relationship, indicating that the nonlinear behavior is associated with the pinning of domain walls on randomly distributed pinning centers. (C) 1998 American Institute of Physics. [S0003-6951(98)01840-3].