Calcium currents were examined in rat intermediate nucleus cells. Two main currents were identified, a transient current and a sustained current. The transient current inactivated over a few tens of milliseconds. The sustained current required seconds to minutes to inactivate, but the inactivation rate was significant even at normal resting potentials. The main conclusion of this study is that any influence on the cell that leads to a sustained change in the resting membrane potential will have a significant effect on the number of activatable calcium channels. Thus, the influx of Ca2+ ions depends not only on the amplitude and duration of the action potential, but also on the short term history of the cell membrane potential.