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.
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