  We present the phenomenological analysis of the spectrum of longitudinal spin
fluctuations in isotropic itinerant electron antiferromagnets with account of
spin anharmonicity giving rise to coupling of transverse and longitudinal
normal modes. The spectrum consists of a quasielastic part forming a central
peak or a dip, depending on temperature and the Landau relaxation rate. Effects
of spin fluctuation coupling also give rise to an inelastic part of the
spectrum which has a form of resonances or antiresonances near the magnon
frequencies related to non-propagating longitudinal excitations.
