The use of sea anemone toxin, veratridine and scorpion toxin which specifically interact with the gating system of the sodium channel and maintain the channel in an open conformation has permitted a study of the mechanism of transport of monovalent cations through the selectivity filter of this channel. The initial rate of 22Na+ influx through the tetrodotoxin-sensitive Na+ channels of excitable cells is dependent upon the external concentrations of Na+ and Na+-substitutes with the following properties. (a) It is saturable at high Na+ concentrations and increases with the external Na+ concentration in a cooperative manner (nH = 1.6). (b) At low external Na+ concentrations (1 mM), it is activated and then inhibited by increasing external concentrations of monovalent cations such as Li+, guanidinium, hydrazinium, hydroxylamine and K+. The activating effect of these cations disappears at higher external Na+ concentrations (10 mM). The experimental data are consistent with a model involving at least two allosteric cation-binding sites per Na+ channel. The binding of monovalent cations to Na+ sites is characterized by a high positive homotropic cooperativity. Most of the work describes the properties of the Na+ channel in neuroblastoma cells. The mechanism has also been shown to be valid for excitable cells of other types and origins.