  The possibility of describing the behavior of a $Q\overline{Q}$ pair in a hot
plasma in terms of an effective potential is investigated. It is shown that as
long as medium effects can be embodied in a gaussian action, like in the QED
case, the $Q\overline{Q}$ propagator obeys a closed temporal evolution equation
whose large-time behavior is governed by an effective potential. The latter,
beside screening, displays also an imaginary part related to collisions.
