  We present here two epochs of Space VLBI observations at 18 cm of the BL-Lac
type object Mkn 501. Thanks to the high resolution of these new data we have
found that the inner jet is centrally brightened at its beginning but becomes
extended and limb brightened at ~ 8 mas from the core. Moreover a comparison
between the two epochs shows the presence of a possible proper motion with
apparent velocity = 6.7c. VLBI data at 6 and 18 cm of the BL-Lac type object
Mkn 421 are also presented. Observational data have been used to constrain the
jet velocity and orientation.
