  Phase transitions in hot and dense matter and the in--medium behavior of
pseudoscalar mesons ($\pi^{\pm}, \pi^0, K^{\pm}, K^0 ,\bar K^0,\eta {and} \eta'
$) are investigated, in the framework of the three flavor Nambu--Jona-Lasinio
model, including the 't Hooft interaction, which breaks the $U_A(1)$ symmetry.
Three different scenarios are considered: zero density and finite temperature,
zero temperature and finite density in quark matter with different degrees of
strangeness, and finite temperature and density. At T=0, the role of strange
valence quarks in the medium is discussed, in connection with the phase
transition and the mesonic behavior. It is found that the appearance of strange
quarks, above certain densities, leads to meaningful changes in different
observables, especially in matter with \beta$ --equilibrium. The behavior of
mesons in the $T-\rho$ plane is analyzed in connection with possible signatures
of restoration of symmetries.
