  We report on transient effects in the microwave second-order response of two
ceramic MgB2 samples. The time evolution of the second-harmonic signal is
investigated for about 500 s after the sample has been exposed to a variation
of the dc magnetic field. We suggest that during the first seconds the response
is determined by diffusive motion of fluxons, while in the time scale of
minutes it is ruled by magnetic relaxation over the surface barrier.
