Systematic investigation of large amplitude ac excitation effects on the polarization-electric-field (P-E) and strain-electric-field (epsilon-E) characteristics of both "hard" and "soft" Pb(Zr,Ti)O-3 ceramics was performed. Upon ac excitation, a gradual disappearance of the shift on P-E response along the E axis (E-int) was observed for the hard piezoelectric; however, no such change was observed for the soft one. With increasing ac excitation cycle number, for a hard piezoelectric the internal dipolar field and mechanical quality factor Q(m) decrease by following an exponential time law, whereas the electromechanical and dielectric properties increase with significant deviation from the time law. It is proposed that additional domain release mechanism may exist for hard piezoelectric, besides the decrease of internal dipolar field upon continuous ac excitation. (c) 2007 American Institute of Physics.