The semiconducting LaMn1-xMgxO3+delta (LMMO) exhibits highly nonlinear, voltage-limiting, conduction (current switching) in the current-voltage (J-E) relationship. The nonlinearity is observed at field strengths less than 50 V/cm and below the magnetic transition temperature (T-c). The current density in the conducting region is very high, reaching tens of A/cm(2). The threshold electric field (E-TH) shifts to lower values in presence of an external magnetic field. No current switching is observed in ferromagnetic metallic, La0.67Ca0.33MnO3+delta (LCMO) specimens. The nonlinearity observed in LMMO cannot be explained on the basis of antiferromagnetic/paramagnetic insulating phases co-existing with ferromagnetic metallic phases below T-c, because such giant clusters are observed in LCMO. Substitution of Mg increases the Mn4+ concentration, however, does not induce a transition to metallic state below T-c even at high hole concentration; instead, displays the behaviour of localisation. Localisation of carriers is due to the increased hole hopping distance (Mn3+-O-Mg2+-O-Mn4+) and by the spin canting. Below T-c, in the canted ferromagnetic state, the carriers can be delocalised by applying an electric field. The electric field enhances the mobility of the carriers and increases the hopping probability. Above E-TH, large number of carriers are delocalised and set in for conduction, yielding high current density. (C) 2002 Elsevier Science B.V. All rights reserved.