Snapshot VLBI observations at 18 cm have been obtained with a global array at three epochs (1980.1, 1981.8, 1987.9) in order to investigate flux density and/or structural variations for a sample of 21 low frequency variable sources (Padrielli et al. 1987 Astron Astrophys. Suppl. Ser., 67, 63; Bondi et al. 1993 in preparation).