The complex dielectric responses of various La-modified lead zirconate titanate ceramic specimens have been measured under various ac drive and dc bias conditions. Investigations were performed for different La concentrations on specimens with Zr/Ti ratios of 40/60 and 65/35. These investigations demonstrated that the influence of increasing ac drive and dc bias are in opposition: increasing drive results in an increase in the dielectric constant, whereas increasing bias results in a decrease. Investigations as a function of La content revealed a strong decrease in the ac-induced nonlinearities with increasing La content in the compositional range near a crossover between micron-sized domains and tweedlike structures. The ac drive dependent dielectric data were then empirically fitted to a power series expansion in the ac field. An anomalous change in sign of the second-order nonlinearity was found for lower La-content compositions near the temperature of the dielectric maximum. It is believed that this change in sign results from a suppression of ferroelectric order and the stabilization of a paraelectric state at temperatures close to the ferroelectric phase transformation. ©1997 American Institute of Physics.