  We studied the radio structure of high-redshift (z>3) quasars with VSOP at
1.6 and 5 GHz. These sources are the most distant objects ever observed with
Space VLBI, at rest-frame frequencies up to ~25 GHz. Here we give an account of
the observations and briefly highlight the most interesting cases and results.
These observations allowed us, among other things, to estimate the mass of the
central black holes powering these quasars, to identify large misalignments
between the milli-arcsecond (mas) and sub-mas scale radio structures, and to
detect apparent superluminal motion at sub-mas scale.

Astrophysics