Steady state to peak Na current ratio (INa,/INapeak) in Myxicola is greater, under some conditions, in internal Cs than in K, indicating less steady state inactivation in Csi. Csi effects are selective for steady state inactivation, with negligible effects on single-pulse inactivation time constants (Th). Mean Th ratios (Csi to Ki) were 1.04 and 1.02 at 0 and 10 mV. Two pulse inactivation time constants were also little affected. Inactivation is blocked in an all or none manner. Ki has little effect on steady state inactivation in the presence of inward INa, with INa/INapeak often declining to zero at positive potentials and independent of external Na concentration from 1/4 to 2/3 artificial sea water (ASW). Cs also has little effect at more negative potentials, but more with either more positive potentials or Na reduction, both reducing inward INa. K effects are evident when Na channel current is outward. A site in the current pathway when occupied selectively blocks inactivation gate closure. As occupancy does not depend significantly on potential, the site must not be very deep into the membrane field. Inactivation gates may associate with these sites on closure. The inactivated state may consist of a positively-charged structure occluding the inner channel mouth.