The history of motion of charge carriers generated in a gap by ionizing radiation is investigated. The study is limited to cases where the field‐induced motion of these carriers overcomes their inertia and diffusion; the motion is thus completely collision‐dominated. The severity of the space‐charge effect is described by a single nondimensional parameter &ggr;. When &ggr; is not much smaller than unity, usual linearization procedures are inapplicable. The nonlinear problem is solved for two limiting cases. In the first, the electric current is carried by the free electrons and ions as originally created. In the second, the electric current is carried by positive and negative ions, the latter formed by electron attachment. The method of characteristics is employed to solve the equations of motion and exact analytical and numerical solutions are presented. It is shown that if &ggr;>2 the nonlinear effects of the induced charges are important and depart appreciably from calculations based on linear approximations.