  We present a nonperturbative dynamical study of $e^+e^-$ production in
proton- nucleus and nucleus-nucleus collisions at SPS energies on the basis of
a covariant transport approach. For p + A reactions the dilepton yield for
invariant masses m $\leq $ 1.2 GeV is found to be dominated by the decays of
the $\eta, \rho, \omega$ and $\Phi$ mesons in line with the findings of the
CERES collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is,
however, dominated by $\pi^+\pi^-$ annihilation due to the high pion densities
achieved. Whereas for `free' meson masses and form factors the experimental
cross section is slightly underestimated for 0.3 GeV $\leq$ m $\leq$ 0.45 GeV,
different medium modifications of the $\rho$-meson appear compatible with
current CERES data.
