The authors propose a new method of describing the early time evolution of a plasma satisfying the ideal non-linear MHD equations. According to this method the velocity field V(x,t) is expanded in a time series the coefficients of which are calculated assuming that at time t=0 the equilibrium MHD equations are satisfied and a prescribed velocity field V0(x) applies on each volume element of the plasma. The main features of this idea are illustrated by studying the evolution of a helical perturbation in a Z-pinch. They show that to fourth order in time no harmonic generation occurs. The results of this example are compared with the predictions of the linearized perturbation theory.