  We study finite temperature effects on dilepton production from pion-pion
annihilation in hot hadronic matter. The softening of the pion dispersion
relation in a medium is found to enhance the production rate of dileptons with
invariant masses in the region of $2m^*_\pi(T)<M<m_\rho$. On the other hand,
the reduction of the pion electromagnetic form factor at finite temperature
leads to a suppression of the dilepton production rate. Including both effects,
we have found that the dilepton yield is slightly enhanced in the invariant
mass region $M= 270\sim 600$ MeV but is suppressed around the vector meson
resonance. We further discuss the relevance of our results to recent
experimental data from the S+Au collisions at CERN/SPS energies by the CERES
collaboration.
