  Motivated by the structure of one-loop vacuum polarization effects in curved
spacetime we discuss a non-minimal extension of the Einstein-Maxwell equations.
This formalism is applied to Bianchi I models with magnetic field. We obtain
several exact solutions of the non-minimal system including those which
describe an isotropization process. We show that there are inflationary
solutions in which the cosmological constant is determined by the non-minimal
coupling parameters. Furthermore, we find an isotropic de Sitter solution
characterized by a "screening" of the magnetic field as a consequence of the
non-minimal coupling.
