  We investigate the production of $e^+e^-$ pairs from $p + Be$ and
nucleus-nucleus collisions from 2 GeV/A to 200 GeV/A within a covariant
transport approach and focus on the dilepton angular anisotropies as a function
of the dilepton invariant mass. Whereas the low mass regime yields information
about the Dalitz decays of the $\Delta, \eta$ and $\omega$, above M $\approx
$0.5 GeV the angular anisotropies provide additional information about the
$\pi^+\pi^- \to \rho^0 \to e^+e^-$ channel in the medium. The anisotropy
coefficient is found to change its sign for M $>$ 0.5 GeV in case of
nucleus-nucleus reactions when increasing the bombarding energy from 2 GeV/A to
200 GeV/A which sheds some light on the $\pi \pi \to \rho$ dynamics.
