  Dielectron mass spectra are examined for various nuclear reactions recently
measured by the DLS collaboration. A detailed description is given of all
dilepton channels included in the transport model UrQMD 1.0, i.e. Dalitz decays
of $\pi^0,\eta,\omega,\eta'$ mesons and of the $\Delta(1232)$ resonance, direct
decays of vector mesons and $pn$ bremsstrahlung. The microscopic calculations
reproduce data for light systems fairly well, but tend to underestimate the
data in $pp$ at high energies and in $pd$ at low energies. These conventional
sources, however, cannot explain the recently reported enhancement for
nucleus-nucleus collisions in the mass region 0.15 GeV<$M_{ee}$<0.6 GeV. Chiral
scaling and $\omega$ meson broadening in the medium are investigated as a
source of this mass excess. They also cannot explain the recent DLS data.
