This is a report on an ongoing project designed to study structural variations on the milliarcsecond scale in four BL Lac-type objects. The observations are made at λ6 cm with a global VLBI network consisting of the combined US and European networks and with some additional antennae like one in South Africa and one in Finland. A typical experiment involves 7–9 stations and produces maps with a resolution (FWHM) of 1–1.5 m.a.s depending on the declination of the source. Seven experiments have been made since the start in 1978, resulting in hybrid maps at three epochs for each of the four main sources: A0 0235+164,0735+178,Mk 421, and 1749+701. The same restoring beam has been used for each source at all epochs. The epoch March 1979 is an exception though, the maps from then were restored with a larger beam corresponding to the resolution on the European baselines only. The results from the epochs 1978–1979 have already been published (Bååth et al. 1981), and papers describing the results from the later epochs are in preparation.