The resistance degradation behaviour of Nd-doped PZT layers with Ag/Pd electrodes was investigated under electric fields of 250–1250 V/cm at temperatures between 425 and 520 °C. Degradation processes caused by high humidity (water films) could thus be avoided. Several phenomena were found in the course of degradation: conductivity variations on a short-time scale are interpreted in terms of field induced oxygen vacancy concentration changes (stoichiometry polarization). The corresponding diffusion coefficient was estimated. Increase of the current by several orders of magnitude during long-time load was associated with cation motion. Moreover, formation of surface precipitates was found on the field-stressed ceramics. The main occurrence of these precipitates close to anodes is in contrast to a conventional electrochemical interpretation. However, precipitates (mostly consisting of silver) are not directly responsible for the high conductivity of degraded PZT layers. Rather, regions exhibiting metal-like conductivity are irreversibly formed in the bulk.