By using an optical interferometer and other more conventional methods, the responses of piezoelectric and dielectric coefficients of lead zirconate-titanate (PZT) ceramics to AC electric fields have been directly investigated. The experimental results demonstrate the importance of electromechanical nonlinearities present in the ferroelectric ceramics. A phenomenological theory, based on a model of G. Arlt et al. (1987), has been extended to describe the extrinsic contribution to piezoelectric, elastic, and dielectric coefficients. This can be attributed to both the linear and nonlinear vibrations of 90° domain walls in tetragonally distorted ferroelectric ceramics. The proposed theory shows qualitative agreement with the experimental results.