This study reports a high-energy density piezoelectric polycrystalline ceramic composition in the system Pb(Zr1-xTix)O-3-Pb[(Zn1-yNiy)(1/3)Nb-2/3]O-3 (PZT-PZNN). Two different Zr/Ti ratios in the PZT system were investigated: 52/48 corresponding to morphotropic phase boundary (MPB) and 56/44 corresponding to the tetragonal phase. The compositions investigated in this study are represented as: 0.9Pb(Zr0.52Ti0.48)O-3-0.1Pb(Zn1/3Nb2/3)O-3 [0.9PZT (52:48)-0.1PZN]+y wt% MnCO3, where y varies from 0 to 0.9 wt% and 0.9Pb(Zr0.56Ti0.44)O-3-0.1Pb[(Zn0.8Ni0.2)(1/3)Nb-2/3]O-3 [0.9PZT (56:44)-0.1PZNN] +y mol% MnO2, where y varies from 1 to 3 mol%. A high-energy density material is characterized by the large magnitude of the product of the piezoelectric voltage constant (g) and the piezoelectric strain constant (d) given as (dg). The condition for obtaining large magnitude of dg was derived to be as |d|=epsilon(n), where epsilon is the permittivity of the material and n is a material constant having a lower limit of 0.5. The d(33).g(33) value of the samples having composition 0.9 PZT (56:44)-0.1 PZNN+2 mol% MnO2 (sintered in two steps at 1100 degrees-1000 degrees C) was found to be 18456.2 x 10(-15) m(2)/N, which, to the knowledge of the authors, is the highest value reported for polycrystalline ceramics. This composition was also found to exhibit a high magnitude of g(33) as 83.1 V.(m.N)(-1), corresponding to the magnitude of n as 1.126.