Piezoelectric properties of Pb(Zr,Ti)O-3 thin films are investigated as a function of the number of bipolar (switching) and unipolar (nonswitching) voltage pulses. The longitudinal piezoelectric coefficient d(33) decreases with bipolar fatigue reflecting the decrease of switchable polarization. Simultaneously, a strong vertical shift of piezoelectric hysteresis loops is observed, which is considered as the buildup of fixed internal polarization due to the pinning of ferroelectric domains in a preferred orientation. Piezoelectric fatigue induced by unipolar (nonswitching) pulses is considerably smaller than the fatigue under bipolar conditions and can be described by the internal bias field which shifts piezoelectric hysteresis loops along the field axis. (C) 1996 American Institute of Physics.