Magnesia powder has an important application for electrical insulation. In order to study the leakage conduction mechanisms, test samples were made by compressing 10 cm long alloy tubes filled with industrial class magnesia between a central electrode rod and the outer casing. The electrical behaviour between the central electrode rod and the outer casing was characterized up to 1200-degrees-C by measurement of the d.c. resistance and the frequency dependence of the a.c. impedance. Three regimes can be identified. Below 700-degrees-C, an activation energy of 1 eV is attributed to second phase(s) containing silicon and calcium which acts as a bypass. Above this temperature ionic conduction in the magnesia grains becomes predominant with an activation energy close to 2 eV. Finally, at even higher temperature above 1000-degrees-C, the activation energy increases further, indicating mixed ionic-electronic conduction. This last regime yields a voltage dependence to the resistance explained by blocking of the ionic component at the electrodes.