We propose that the mass of the eta' meson is a particularly sensitive
probe of the properties of finite energy density hadronic matter and
quark-gluon plasma. We argue that the mass of the eta') excitation in
hot and dense matter should be small, and, therefore, that the eta' pr
oduction cross section should be much increased relative to that for p
p collisions. This may have observable consequences in dilepton and di
photon experiments.