  We present the results of 5 GHz VLBI observations of a compact steep spectrum
source 3C 138. The data are consistent with the western end being the location
of the central activity. The observed offset between different frequencies in
the central region of 3C 138 can be accounted for by a frequency dependent
shift of the synchrotron self-absorbed core. Our new measurements confirm the
existence of a superluminal motion, but its apparent velocity of 3.3c is three
times slower than the reported one. This value is consistent with the absence
of parsec-scale counter-jet emission in the inner region, but seems still too
high to allow the overall counter-jet to be seen in terms of Doppler boosting
of an intrinsically identical jet. Either an interaction of jet with central
dense medium, or an intrinsically asymmetrical jet must be invoked to reconcile
the detected superluminal speed with the observed large scale asymmetry in 3C
138.
