Three undoped lead zirconate titanate (PZT) ceramics were prepared with compositions close to the morphotropic phase boundary: Pb(Zr0.50Ti0.50)O-3, Pb(Zr0.52Ti0.48)O-3, and Pb(Zr0.54Ti0.46)O-3. Internal friction Q(-1) and shear modulus G were measured versus temperature from 20 degreesC to 500 degreesC. Experiments were performed on an inverted torsional pendulum at low frequencies (0.1, 0.3, and 1 Hz). The ferroelectric-paraelectric phase transition results in a peak (P-1) of Q(-1) correlated with a sharp minimum M-1 of G. Moreover the Q(-1)(T) curves show two relaxation peaks called R-1 and R-2 respectively, correlated with two shear modulus anomalies called A(1) and A(2) on the G(T) curves. The main features of the transition P-1 peak are studied, they suggest that its behavior is similar to the internal friction peaks associated with martensitic transformation. The relaxation peak, R-1 and R-2 are both attributed to motion of domain walls (DWs), and can be analyzed by thermal activated process described by Arrhenius law. The R-2 peak is demonstrated to be due to the interaction of domain walls and oxygen vacancies because it depends on oxygen vacancy concentration and electrical polarization. However, the R-1 peak is more complex; its height is found to be increased as stress amplitude and heating rate increase. It seems that the R-1 peak is influenced by three mechanisms: (i) relaxation due to DW-point defects interaction, (ii) variation of domain wall density, and (iii) domain wall depinning from point defect clusters. (C) 2002 American Institute of Physics.