[en] We have investigated the chemical structure of PbMg_1/3Nb_2/3O₃ (PMN) relaxor ferroelectric at room temperature under hydrostatic pressures up to 8.37 GPa and at ambient pressure as a function of temperature. The refinement of the crystal structure shows that the amplitude of the displacements of the Pb ions from the (0 0 0) position decreases with increasing pressure. On the other hand, the thermal displacement parameters of Mg/Nb ions are almost insensitive to pressure. The decrease in the Pb displacements correlates with an increase in the anisotropy in the thermal displacement parameter of the O ions. These results are in agreement with those obtained in PbMg_1/3Ta_2/3O₃ (PMT) and show further evidence that the Pb displacements play an important role in the relaxor behavior. The aim of this study is to clarify the microscopic origin for the anomalous pressure effects observed in relaxors as well as to provide new data for the understanding of this class of materials