Non-Ohmic effects in the magnetoresistance of n-type GaSb epitaxial layers are investigated as the magnetic field is varied through a critical Bc value (Bc=9.1T) where the electron system undergoes a metal-insulator transition. The low temperature Ohmic conductivity is mainly due to either free or localized electrons, depending on B, so that different pictures are proposed to explain the observed nonlinear behaviors when the electric field is increased. The experiment is made possible by the availability of samples with electron densities slightly lower than 1016cm−3, grown by molecular beam epitaxy, in order that the electron gas can be driven to the metal-insulator transition at an experimentally accessible value of B. Both the longitudinal (B‖I) and transverse (B⊥I) configurations are investigated.