Electron paramagnetic resonance of Mn^{2+} in La_{2}Zn_{3}(NO_{3})_{12} • 24H_{2}O single crystals is studied at ≈ 9.45 GHz. Mn^{2+} substitutes for Zn^{2+} sites. The spin-Hamiltonian parameters are evaluated at 290 K. The superposition model has been applied to the zero-field splitting parameter D for Mn^{2+} in Ce_{2}Mg_{3}(NO_{3})_{12} • 24H_{2}O and La_{2}Mg_{3} (NO_{3})_{12} • 24D_{2}O at both sites. It is shown that the calculated values of D are in agreement with the experimental values if local relaxation effects are taken into account.