When a constant electric field is applied to a dielectric, the current density per unit field decreases with time from an ''initial conductivity'' to a ''final (or steady-state) conductivity.'' A study is made of this time-dependent polarization effect for ''pure'' NaCl crystals in the range from 50°to 200°C. The validity of Ohm's law and the superposition principle is demonstrated for these crystals; this establishes the linearity of the formal equation which relates polarization, electric field, and their time derivatives. Also studied are the effect of prolonged current flow, the effect of deliberate introduction of an air gap between the crystal and one of its electrodes, and the effects of impurities, deformation, x-ray irradiation, and annealing. It is concluded that the results are not consistent with the well-known space-charge polarization theory of Joffé, according to which the buildup of space charge occurs because of blocking of the current carriers at one or both electrodes. Rather, it is necessary to regard the time-dependent polarization as a dielectric relaxation phenomenon. According to this viewpoint, the final conductivity, and not the initial value, represents the true ionic conductivity. Possible relaxation mechanisms are discussed in terms of defect clusters and charged jogs on dislocations. © 1963 The American Institute of Physics.