  We revisit the theory of magnetoresistance for a system of nanoscopic
magnetic granules in metallic matrix. Using a simple model for the spin
dependent perturbation potential of the granules, we solve Boltzmann equation
for the spin dependent components of the non equilibrium electronic
distribution function. For typical values of the geometric parameters in
granular systems, we find a peculiar structure of the distribution function of
conduction electrons, which is at variance with the two-current model of
conduction in inhomogeneous systems. Our treatment explicitly includes the
effects of dipolar correlations yielding a magnetoresistance ratio which
contains, in addition to the term proportional to the square of uniform
magnetization (< {\boldsymbol \mu} >), a weak anisotropic contribution
depending on the angle between electric and magnetic fields, and arising from
the anisotropic character of dipolar interactions.
