Saturable, high affinity binding of tritium-labeled saxitoxin ([3H]STX) to axolemma-enriched membranes from white matter of bovine brain was identified. The apparent [3H]STX equilibrium dissociation constant (Kd*) was strongly affected by the cationic environment:choline ion had little effect; cesium ion increased the mammalian axolemma Kd* in a simple competitive manner. In contrast, sodium ion more dramatically increased the Kd*--this effect was highly cooperative between 75 and 200 mM sodium (Hill coefficient of 2.85). The cooperativity is most pronounced at the normally expected [sodium] external to the axon in the mammalian central nervous system. This sodium-specific cooperative modification of the STX binding site (the hypothetical "ion selectivity filter" of the axonal Na+ gate) may be indicative of some as yet undefined regulatory mechanism of the Na+ gate in mammalian myelinated axons.