Na currents and Na-current fluctuations were measured in myelinated frog nerve fibres to study interactions between monovalent externally applied cations and the binding of the Na-channel blockers tetrodotoxin (TTX) or saxitoxin (STX). Adding ll0mM NaC1 to Ringer's solution increased the maximum peak Na conductance by a factor of 2.51 in the presence of 12 nM TTX and by a factor of 2.43 in the presence of 4 nM STX. According to the analysis of Na-current fluctuations this increase of the Na conductance is mainly caused by an increase of the number N of unblocked Na channels per node, while the conductance of a single channel saturates in the hyperosmolar solutions. The increase of N is interpreted by displacement of TTX or STX from Na channels by external Na+. Relief of TTX blockage was also observed by adding 110raM chloride salts of Li +, hydrazine +, guanidine + and K + to Ringer, but not in Ringer+ll0mM tetramethylammonium chloride or 250 mM sucrose. The increase of N by the external cations is a saturating function of the permeability of the Na channel to these ions. The results are interpreted by a toxin receptor in a superficial prefilter to the Na channel, which contributes to cation discrimination at the outer channel region.