The Ni\H2SO4 system is a well-known oscillator under galvanostatic conditions. New experimental results obtained forcing the Ni\1 M H2SO4 electrochemical oscillator under galvanostatic conditions are presented. First a simple dissolution-passivation model explaining potential oscillations under galvanostatic conditions is used to predict the possible behaviour of experimental systems. The non-linear dynamic behaviour of the model is studied numerically near the Hopf bifurcation. The classical behaviour for forced self-oscillating systems, such as periodic or quasi-periodic behaviour, may occur when the current density is modulated by a sinusoidal signal. A phase portrait showing the main Arnold tongues where phase locking occurs is plotted. Next, several types of behaviour theoretically predicted for forced oscillators are observed experimentally for the Ni\ 1 M H2SO4 system, for example phase locking, bi-periodic behaviour, etc. Finally the consequences of bi-periodic bebaviour on impedance measurements are pointed out experimentally. (C) 2004 Elsevier B.V. All rights reserved.