  Density and thermal corrections to the mass of the pions are studied in the
framework of the SU(2) low energy effective chiral lagrangian, in terms of the
isospin chemical potential $\mu_\tin{I}$. We concentrate the discussion in the
region where the isospin chemical potential (absolute value) becomes bigger
than the pion mass at zero temperature and density, i.e. in the phase where the
condensed $\pi^-$ phase appears (for negative chemical potential). We are able
to calculate the thermal and density evolution of masses in the limits where
$|\mu_{I}|\gg m$ and where $|\mu_{I}| \gtrsim m$. We also identified the phase
transition curve.
