  We present a nonperturbative dynamical study of $\mu^+\mu^-$ production in
proton-nucleus and nucleus-nucleus collisions at SPS energies on the basis of
the covariant transport approach HSD. For p + W reactions at 200 GeV bombarding
energy the dimuon yield for invariant masses m $\leq $ 1.6 GeV is found to be
dominated by the decays of the $\eta, \rho, \omega$ and $\Phi$ mesons. For 200
GeV/A S + W collisions, however, the dimuon yield shows an additional large
contribution from $\pi^+\pi^-$, $K^+K^-$ and $\pi \rho$ channels. We find that
for `free' meson masses and form factors the experimental cross section is
clearly underestimated for S + W in the invariant mass range 0.35 GeV $\leq$ m
$\leq$ 0.65 GeV and that the HELIOS-3 data can only be reproduced within a
hadronic scenario, if the $\rho/\omega$-meson mass drops with baryon density.
This finding suggests a partial restoration of chiral symmetry in S + W
collisions at SPS energies.
