  The $s-$wave meson-baryon scattering amplitude is analyzed for the
strangeness $S=-1$ and isospin I=0 sector in a Bethe-Salpeter coupled channel
formalism incorporating Chiral Symmetry. Four two-body channels have been
considered: $\bar K N$, $\pi \Sigma $, $\eta \Lambda $, $ K \Xi$. The needed
two particle irreducible matrix amplitude is taken from lowest order Chiral
Perturbation Theory in a relativistic formalism. Off-shell behaviour is
parameterized in terms of low energy constants, which outnumber those assumed
in previous works and provide a better fit to the data. The position of the
complex poles in the second Riemann sheet of the scattering amplitude determine
masses and widths of the $\Lambda (1405)$ and $\Lambda(1670)$ resonances which
compare well with accepted numbers.
