  Nucleon spin fluctuations in a dense medium reduce the ``naive'' values of
weak interaction rates (neutrino opacities, neutrino emissivities). We extend
previous studies of this effect to the degenerate case which is appropriate for
neutron stars a few ten seconds after formation. If neutron-neutron
interactions by a one-pion exchange potential are the dominant cause of neutron
spin fluctuations, a perturbative calculation of weak interaction rates is
justified for $T\alt 3m/(4\pi\alpha_\pi^2)\approx 1 MeV$, where $m$ is the
neutron mass and $\alpha_\pi\approx15$ the pion fine-structure constant. At
higher temperatures, the application of Landau's theory of Fermi liquids is no
longer justified, i.e. the neutrons cannot be viewed as simple quasiparticles
in any obvious sense.
