The paper is mainly devoted to an analysis of problems set by the investigation of solutions of transport equations, dealing with transient nonlinear conductivity resulting from space-charge build-up. Prior to deriving a general form of solutions obtainable when diffusion is neglected, a brief review is presented of one-carrier type results available from the literature. Emphasis is laid on the improper character of approximate methods using the separation of space and time variables. The problem of interactions between space-charge build-up and dielectric relaxation is tackled through introducing the Boltzmann-Hopkinson principle of superposition. It is then shown that the resulting electric field is readily obtained, as soon as a solution of the transport equations is known.