  The s-wave isoscalar amplitude for pion-pion scattering in a nuclear medium
is evaluated using a nonperturbative unitary coupled channels method and the
standard chiral Lagrangians. The method has proved successful to describe the
pion-pion properties in the scalar isoscalar channel up to 1.2 GeV giving rise
to poles in the t matrix for the f0(980) and the sigma. The extension of the
method to the nuclear medium implies not only the renormalization of the pions
in the medium, but also the introduction of interaction terms related to
contact terms in the pion-nucleon to pion-pion-nucleon interaction. Off shell
effects are also shown to be important leading to cancellations which reduce
the coupled channel integral equations to a set of algebraic equations. As the
density increases we find a reduction of strength below the $\sigma$ region and
a certain accumulation of strength at energies around pion threshold. Our
results, based on chiral Lagrangians, provide similar results to those obtained
with phenomenological models which impose minimal chiral constraints.
