1. Two temperature regions, differing in the magnitude of the activation energy of discharging of nickel ions, were established. At low temperatures the activation energy decreases from 15 to 6.2 kcal/mole with increasing overvoltage, while in the high-temperature region the activation energy does not depend on the value of the over-voltage and constitutes 4 kcal/mole. 2. In the temperature region in which the activation energy does not depend on the overvoltage, the limiting step is the delivery of the substance to the growing portions of the electrode. In the region of high values of the activation energy, the polarization is chemical, due to the inhibiting action of foreign particles. 3. The hypothesis is advanced that the small value of the exchange current of metals of the iron group is due to passivation of the electrode surface. The true value of the exchange current is significantly greater than that cited in the literature. The value of the activation energy atη0, calculated on the basis of the experimental data, is about 16.9 kcal/mole.