Phase transitions in hot and dense matter and the in--medium behavior of pseudoscalar mesons ($��^{\pm}, ��^0, K^{\pm}, K^0 ,\bar K^0,��{and} ��' $) 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 ��$ --equilibrium. The behavior of mesons in the $T-��$ plane is analyzed in connection with possible signatures of restoration of symmetries.

33 pages, 12 figures, PRC version