Theoretical studies of the solar-terrestrial interaction often are carried out assuming that steady state conditions are applicable and with the a priori assumption that any specific change in the input conditions will produce magnetospheric and ionospheric responses which are repeatable. In this paper we shall first demonstrate that the variability of the interplanetary magnetic field (IMF) is sufficiently large that, under normal circumstances, the assumption of steady state conditions is questionable. We shall further show that the response of the magnetosphere-ionosphere current systems to changes in interplanetary conditions is dependent on the prior state of the system. In this respect it will be shown that the strength of the driven system currents which grow in response to a southward turning of the IMF is dependent on the level of magnetospheric activity prior to that southward turning (as quantified by the AL index of geomagnetic activity). ¿ American Geophysical Union 1988