  We present a dynamical study of $e^+e^-$ and $\mu^+ \mu^-$ production in
proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis
of the covariant transport approach HSD employing a momentum-dependent
$\rho$-meson spectral function that includes the pion modifications in the
nuclear medium as well as the polarization of the $\rho$-meson due to resonant
$\rho$$-N$ scattering. We find that the experimental data from the CERES and
HELIOS-3 Collaborations can be described equally well as within the dropping
$\rho$-mass scenario. Whereas corresponding dilepton $q_T$-spectra are found to
be very similar, the inclusive dilepton yield in the invariant mass range $0.85
\leq M \leq 1.0$ GeV should allow to disentangle the two scenarios
experimentally.
