  The behavior of neutral pseudoscalar mesons $\pi^0, \eta$ and $\eta'$ in hot
and dense matter is investigated, in the framework of the three flavor
Nambu-Jona-Lasinio model. Three different scenarios are considered: zero
density and finite temperature, zero temperature and finite density in a flavor
asymmetric medium with and without strange valence quarks, and finite
temperature and density. The behavior of mesons is analyzed in connection with
possible signatures of restoration of symmetries. In the high density region
and at zero temperature it is found that the mass of the $\eta'$ increases, the
deviation from the mass of the $\eta$ being more pronounced in matter without
strange valence quarks.
