The electrical conductivity of ceramics of composition Ba(Ti_1−xMg_x)O_3−x: 0.00003 ≤ x ≤ 0.01 increases with time on application of a small dc bias voltage in the range 1 to 50 V cm⁻¹ and at temperatures in the range 200 to 600 °C. The conductivity increase is reversible on removal of the dc bias. This low field effect is not observed in undoped BaTiO₃, is intrinsic, is a property of the bulk material, is independent of atmosphere and electrode material, is not associated with charge injection into the BaTiO₃ conduction band and differs from those of both varistors and memristors. The origin of the effect is attributed to a two-level electronic structure in a defect complex that contains Mg located on a Ti site, giving rise to underbonded oxide ions in the vicinity of the defect complex that are readily ionised on application of a dc bias. Conduction is predominantly p-type and therefore, holes located on oxygen, as O⁻ ions, are more mobile than both electrons in Ti 3d levels and oxygen vacancies.