  The energy spectrum of nucleons in high-density nuclear matter is
investigated in the framework of relativistic meson-nucleon many-body theory,
employing the $1/N$ expansion method. The coupling of the nucleon with the
particle-hole excitations in the medium flattens the spectra in the vicinity of
the Fermi surface. The effect grows logarithmically for increasing density and
eventually leads to instability of the normal state. The validity of the
mean-field theory at high density is criticized.
