[en] We measured the magnetic relaxation in the high-T_c superconductor YBa₂Cu₃O_x at a wide range of temperatures and applied fields. The relaxation measurements were performed on single-crystal, melt-textured, and sintered samples. We found that magnetic relaxation depends strongly on the microstructure and crystal orientation. By combining Bean's critical-state model and the Anderson-Kim flux-creep theory, we used a previously developed expression to interpret the magnetic relaxation behavior and the dependence of the field penetration on the sample size. We also investigated the relationship between the flux-creep-activation pinning energy and the decay of the persistent current in the single-crystal sample of YBa₂Cu₃O_x