  The $s-$wave meson-baryon scattering is analyzed for the strangeness $S=-1$
and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism
incorporating Chiral Symmetry. Four channels have been considered: $\pi
\Sigma$, $\bar K N$, $\eta \Lambda$ and $K \Xi$. The required input to solve
the Bethe-Salpeter equation is taken from lowest order Chiral Perturbation
Theory in a relativistic formalism. There appear undetermined low energy
constants, as a consequence of the renormalization of the amplitudes, which are
obtained from fits to the $\pi\Sigma\to\pi\Sigma$ mass-spectrum, to the elastic
$\bar K N \to \bar K N$ and $ \bar K N\to \pi \Sigma$ $t$--matrices and to the
$ K^- p \to \eta \Lambda$ cross section data. The position and residues of the
complex poles in the second Riemann Sheet of the scattering amplitude determine
masses, widths and branching ratios of the $S_{01}-$ $\Lambda$(1405) and
$-\Lambda$(1670) resonances, in reasonable agreement with experiment. A good
overall description of data, from $\pi \Sigma$ threshold up to 1.75 GeV, is
achieved despite the fact that three-body channels have not been explicitly
included.
