The propagation of long magnetohydrodynamic waves (with wavelength lambda)
in a plasma characterized by small scale (of the order of a), large amplitu
de nonuniformities of its density and of the magnetic held is investigated
in conditions where lambda >> a. The introduction of a suitable averaging p
rocedure over an intermediate scale allows one to obtain the relevant dispe
rsion relation for the long waves which manifests new properties with respe
ct to those of a uniform plasma. Moreover, it is shown that, if the plasma
nonuniformities are spatially distributed with a broad spectrum, within a f
inite interval of numerical values, a collisionless mechanism of damping ma
y affect the slow mode. The results of this work are of concern both For as
trophysical and laboratory plasmas.