Decay-current and conventional dielectric measurements have been performed on single-crystal samples of the disordered perovskite Pb(Mg_1/3Nb_2/3)O₃(PMN). Low-frequency dielectric data of Colla and co-workers(1992) on the same system are critically reanalysed and compared to our results. In contrast to previously proposed spin-glass or master-plot approaches, we suggest a phenomenological description in terms of alpha - and beta -relaxations: a relatively high-frequency beta -process takes over when the characteristic alpha -relaxation time becomes extremely long. The continuous slowing down of polar motion distinguishes PMN from related compounds such as Pb(Zn_1/3Nb_2/3)O₃ for which, in contrast to PMN, there is evidence of astructural phase transition.