  The production and propagation of mesons ($\pi, \eta, \rho, \omega, \Phi, K,
\bar{K}, J/\Psi$) in proton-nucleus and nucleus-nucleus collisions from 1 - 200
GeV/u is studied within the covariant transport approach HSD, which explicitly
allows to investigate selfenergy effects of the hadrons at finite baryon
density. Whereas the experimental pion and $K^+$ spectra can be described
without introducing any selfenergies for the mesons, the $K^-$ yield in Ni + Ni
collisions is underestimated by a factor of 5 - 7 at 1.66 and 1.85 GeV/u.
However, introducing density dependent antikaon masses in line with effective
chiral Lagrangians a satisfactory agreement with the data is achieved. A
dropping of the $\rho$-meson mass with baryon density, as suggested by QCD
sumrule studies, is proposed to explain the dilepton spectra for S + Au and Pb
+ Au at SPS energies, which indicates independently that a partial restoration
of chiral symmetry might be found already in the present experiments.
