  The density corrections, in terms of the isospin chemical potential $\mu_I$,
to the mass of the pions are studied in the framework of the SU(2) low energy
effective chiral lagrangian. The pion decay constant $f_{\pi}(T, \mu_{I})$ is
also analized. As a function of temperature for $\mu_I =0$, the mass remains
quite stable, starting to grow for very high values of $T$, confirming previous
results. However, there are interesting corrections to the mass when both
effects (temperature and chemical potential) are simultaneously present. At
zero temperature the $\pi ^{\pm}$ should condensate when $\mu_{I} = \mp
m_{\pi}$. This is not longer valid anymore at finite $T$. The mass of the
$\pi_0$ acquires also a non trivial dependence on $\mu_I$ due to the finite
temperature.
