Voltage clamp experiments were done on single nodes of Ranvier to study the inhibition of the sodium permeability by tetrodotoxin (TTX). Equilibrium results could be excellently fitted on the assumption that a sodium channel is blocked when one toxin molecule binds to it, the equilibrium dissociation constant, K T of this reaction being 3.6 nM at 20 °G. Onset and offset of block could be quantitatively interpreted to be determined by the rates of the TTX-channel reaction whose average constants, at room temperature, were 3 x 10 6 m -1 s -1 for the association (k 1 ) and 1.4 x 10 -2 s -1 for the dissociation (k 2 ). The dependence of the constants on temperature could be described by Arrhenius plots yielding activation energies of 29.3, 85.5 and 41.0 (57.3) kJ/mol for K T , k 2 and k 1 (k 1 derived from onset alone), respectively. At low pH the relative TTX effect was clearly less than at neutral pH. These results could be explained by a model involving the competition of TTX and protons for the same’receptor to which protons bind as a function of membrane potential.