  Dilepton production from both pion-pion and kaon-antikaon annihilation in
heavy-ion collisions is studied using the relativistic transport model. The
formation of a rho meson from pion-pion annihilation and a phi meson from
kaon-antikaon annihilation, their propagation in the medium, and their decay
into dileptons are explicitly treated. Including the medium modifications of
the masses and widths of vector mesons as predicted by the QCD sum-rule
calculations, we study their effects on the dilepton invariant mass spectra
from heavy-ion collisions at SIS/GSI energies.
