  A significant drop of the vector meson masses in nuclear matter is observed
in a chiral SU(3) model due to the effects of the baryon Dirac sea. This is
taken into account through the summation of baryonic tadpole diagrams in the
relativistic Hartree approximation. The appreciable decrease of the in-medium
vector meson masses is due to the vacuum polarisation effects from the nucleon
sector and is not observed in the mean field approximation.
