  We study a simplified model of the electronic structure of compounds of the
type of La$_{1-x}$Sr$_x$MnO$_3$. The model represents each Mn$^{4+}$ ion by a
spin S=1/2, on which an electron can be added to produce Mn$^{3+}$. We include
two strong intratomic interactions in the Hamiltonian: exchange ($J$% ) and
Coulomb ($U$). Finally, to represent the effect of Sr substitution by La in a
simple way, we include a distribution of diagonal energies at the Mn sites.
Then we use Green function techniques to calculate a mobility edge and the
average density of states. We find that according to the amount of disorder and
to the concentration of electrons in the system, the Fermi level can cross the
mobility edge to produce a metal to insulator transition as the magnetization
decreases (increase of temperature). If the disorder is large, the system
remains insulating for all concentrations. Concentrations near zero or one
favor the insulating state while intermediate values of concentration favor the
metallic state.
