The design of ferroelectric/piezoelectric smart structures is limited by the accuracy to which the material properties of the sensor/actuator materials are determined. In particular, it is important to understand the effects of losses, dispersion, and the non-linearities that become significant when large fields are applied. This paper presents the small signal properties, including losses of Motorola PZT 3203 HD, a typical piezoelectric material, and it reports on the field dependence of the material constants for large electric fields. A set of PZT 3203 HD unloaded resonators manufactured by Motorola was cut to specifications outlined in the IEEE Standard on Piezoelectricity Std 176-1987, to ensure the appropriate boundary conditions of each resonance mode. Quasi-static measurements were performed on some of the samples at various field levels above and below the coercive field of the material. The impedance/admittance spectra of the resonators were measured for different values of the DC bias field. In both cases the average slope as a function of field, which is a measure of the piezoelectric or the dielectric constant, was found to increase linearly with the maximum field applied. The values of the material constants determined from the DC biased spectra were found to be smaller by a factor of 4 - 6. This is attributed to differences in the nature of the measurements. The quasistatic measurements are done at high field and low frequency and involve irreversible domain switching. The DC bias measurement is at high frequency and the AC measurement field is much smaller and the domain motion is reversible.