  Dilepton and photon production in heavy-ion collisions at SPS energies is
studied in the relativistic transport model that incorporates self-consistently
the change of hadron masses in dense matter. It is found that the dilepton
spectra in proton-nucleus reactions can be well described by the conventional
mechanisms of Dalitz decay, primary vector meson decay, decay of charmed
mesons, and the initial Drell-Yan processes. However, to provide a quantitative
explanation of the observed dilepton spectra in central heavy-ion collisions
requires contributions other than these direct decays and also various medium
effects. Introducing a decrease of vector meson masses in hot dense medium, we
find that the low-mass dilepton enhancement can be satisfactorily explained.
Furthermore, to explain the intermediate-mass dilepton enhancement in heavy-ion
collisions, secondary processes such as $\pi a_1\to l{\bar l}$ are found to be
very important. Finally, the single photon spectra in our calculations with
either free or in-medium meson masses do not exceed the upper limit measured by
the WA80 Collaboration.
