  We study the production of massive fermions in arbitrary vector and
axial-vector classical backgrounds using effective action techniques. A
perturbative calculation shows the different features of each field and in
particular it is seen that pure temporal axial fields can produce particles
whereas it is not possible for a pure vector background. We also analyze from a
non-perturbative point of view a particular configuration with constant
electric and axial fields and show that the presence of the axial background
inhibits the production from the electric field.
