  We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and
Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures
are present in three out of four sources on scales of ~100 pc to ~1 kpc, and
the 20-mas beam of the EVN enables us to resolve details within the radio
structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data
combined with the EVN data to improve the sensitivity to extended emission. We
find an unresolved flat-spectrum core in Mrk 3, which we identify with the
hidden Seyfert 1 nucleus in this object, and we also see marked differences
between the two highly-collimated radio jets emanating from the core. The
western jet terminates in a bright hotspot and resembles an FRII radio
structure, whilst the eastern jet has more in common with an FRI source. In Mrk
463E, we use the radio and optical structure of the source to argue that the
true nucleus lies approximately 1 arcsec south of the position of the radio and
optical brightness peaks, which probably represent a hotspot at the working
surface of a radio jet. The EVN data also provide new evidence for a 100-pc
radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However,
the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits
of resolution (~10 pc). We discuss the range of radio source size and
morphology which can occur in the nuclei of Seyfert galaxies and the
implications for Seyfert unification schemes and for radio surveys of large
samples of objects.
