We report on in situ electromigration and potentiometry measurements on single-crystalline Ag nanowires under ultra-high vacuum (UHV) conditions, using a four-probe scanning tunnelling microscope (STM). The Ag nanowires are grown in place by self-organization on a 4° vicinal Si(001) surface. Two of the four available STM tips are used to contact the nanowire. The positioning of the tips is controlled by a scanning electron microscope (SEM). Potentiometry measurements on an Ag nanowire were carried out using a third tip to determine the resistance per length. During electromigration measurements current densities of up to 1 × 10(8) A cm(-2) could be achieved. We use artificially created notches in the wire to initiate electromigration and to control the location of the electromigration process. At the position of the notch, electromigration sets in and is observed quasi-continuously by the SEM.