In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, Greece, was remeasured using the GPS technique. This network was established in 1979, in the epicentral area of the 1978 earthquake (Ms=6.5), and was measured using triangulation techniques seven times between 1979 and 1989. The comparison between 1979 and 1994 data shows that this part of the Mygdonian graben experienced about 8 cm of N-S horizontal extension. Most of the extension is concentrated in a narrow (one or two kilometers) E-W zone, located on the southern boundary of the graben. Furthermore, the analysis of repeated triangulation surveys suggests a rate of extension of 5.7¿1.3 mm/y, that remained constant with time between 1979 and 1994. This extension could result from long-term postseismic relaxation processes, or from continuous aseismic slip within the graben. ¿ 1997 American Geophysical Union