  Using a relativistic transport model to describe the expansion of the
fire-cylinder formed in the initial stage of heavy-ion collisions at SPS/CERN
energies, we study the production of dileptons with mass below about 1 GeV from
these collisions. The initial hadron abundance and their momentum distributions
in the fire-cylinder are determined by following the general features of the
results from microscopic models based on the string dynamics and further
requiring that the final proton and pion spectra and rapidity distributions are
in agreement with available experimental data. For dilepton production, we
include the Dalitz decay of $\pi ^0$, $\eta$, $\eta^\prime$, $\omega$ and $a_1$
mesons, the direct decay of primary $\rho ^0$, $\omega$ and $\phi$ mesons, and
the pion-pion annihilation that proceeds through the $\rho^0$ meson, the
pion-rho annihilation that proceeds through the $a_1$ meson, and the
kaon-antikaon annihilation that proceeds through the $\phi$ meson. We find that
the modification of vector meson properties, especially the decrease of their
mass due to the partial restoration of chiral symmetry, in hot and dense
hadronic matter, provides a quantitative explanation of the recently observed
enhancement of low-mass dileptons by the CERES collaboration in central S+Au
collisions and by the HELIOS-3 collaboration in central S+W collisions.
