A new approach to nonlinear electronic transport in solids is described briefly and is applied to the discussion of combined acoustic and optical phonon. scattering. It is found that although a relatively flat region may exist in the currenit-versus-field curve if the scattering by optical phonon is strong, the saturation of the current density, which occurs in the case of pure optical scattering, disappears and the acoustic phon-ons are always important at high current density.