We investigated the reversible electromigration in Pd-Pt nanobridges by means of in situ electron microscopy. Real-time nanometer-scale imaging with scanning transmission electron microscopy was used to determine the material transport. For high current densities (3-5 x 10(7) A cm(-2)), material transport occurs from the cathode towards the anode side, indicating a negative effective charge. The electromigration is dominated by atom diffusion at grain boundaries on the free surface. The reversal of material transport upon a change of the electric field direction could be the basis of a memristor.