  The influence of time dependent medium modifications of low mass vector
mesons on dilepton production is investigated using nonequilibrium quantum
field theory. By working in the two-time representation memory effects are
fully taken into account. For different scenarios the resulting dilepton yields
are compared to quasi-equilibrium calculations and remarkable differences are
found leading to the conclusion that memory effects can not be neglected when
calculating dilepton yields from heavy ion collisions. Emphasis is put on a
dropping mass scenario since it was recently claimed to be ruled out by the
NA60 data. Here the memory effects turn out to be particularly important.
