  We discuss the microwave second-harmonic generation in high-density bulk
MgB2, prepared by the reactive liquid Mg infiltration technology. The intensity
of the harmonic signal has been investigated as a function of temperature and
amplitudes of the DC and microwave magnetic fields. The results are discussed
in the framework of a phenomenological theory, based on the two-fluid model,
which assumes that both the microwave and static magnetic fields, penetrating
in the surface layers of the sample, weakly perturb the partial concentrations
of the normal and superconducting fluids. We show that, in order to account for
the experimental results, it is essential to suppose that in MgB2 the densities
of the normal and condensed fluids linearly depend on the temperature.
