The paper deals with the assessment of the influence of soil ionization on the transient current induced along the horizontal grounding electrode. Transient response of the grounding electrode buried in a lossy half-space is determined via the analytical solution of the corresponding Pocklington equation in the time domain. The electrode is excited by an equivalent current source representing the lightning strike current. Presence of the earth-air interface is taken into account via simplified reflection coefficient arising from the Modified Image Theory (MIT). The influence of the soil ionization is taken into account as an increase of soil conductivity when critical value of the electric field is exceeded in the vicinity of the electrode. The calculations have been performed for different steady-state conductivities of the soil, various electrode lengths and different types of lightning strikes. The results for the transient current have been compared to the results obtained by calculation without influence of soil ionization.