Inactivated states can be modified by binding of molecules both to the outside and to the inside of the pore of ion channels. Here, we explore the modulation of ultra-slow inactivation (Ius) by binding of Cd2+ to a cysteine engineered into the selectivity filter of rNav 1.4 channels (K1237C). 30 mM Cd 2+ decreased the time constant of recovery from Ius from 177.6±21.3 s to 31.3±1.3 s. Recovery from Ius was only accelerated if Cd2+ was added to the bath solution during recovery from Ius, but not when the channels were exposed to Cd2+ during entry into Ius. The data are consistent with a kinetic model in which Cd2+ binds with high affinity to a slow inactivated state (Is) which is transiently occupied during recovery from Ius. Binding of lidocaine (300 mM) to the internal vestibule of K1237C channels reduced the number of channels recovering from Ius suggesting that Ius is associated with a conformational change of the internal vestibule of the channel. We propose a physical model in which Ius arises from a widening of the selectivity filter region which, through an interaction with the DIV S6 segment, produces a collapse of the internal vestibule. By contrast, Is has been suggested to occur by a "collapse" of the outer vestibule. Cd2+ may promote this collapse, thereby hastening recovery from Ius. Support: Austrian Science Fund P17509-B11