  We apply the momentum integrated Boltzmann transport equation to study the
time evolution of various quark flavours in the central region of
ultra-relativistic heavy ion collisions. The effects of thermal masses for
quarks and gluons are incorporated to take into account the in-medium
properties of these ingredients of the putative quark gluon plasma. We find
that even under very optimistic conditions, complete chemical equilibration in
the quark gluon plasma appears unlikely.
