The binding of saxitoxin, a specific inhibitor of the sodium conductance in excitable membranes, has been measured in giant axons from the squid, Loligo pealei. Binding was studied by labeling saxitoxin with tritium, using a solvent-exchange technique, and measuring the toxin uptake by liquid scintillation counting. Total toxin binding is the sum of a saturable, hyperbolic binding component, with a dissociation constant at 2--4 degrees C of 4.3 +/- 1.7 nM (mean SE), and a linear, nonsaturable component. The density of saturable binding sites is 166 +/- 20.4 micrometers-2. From this density and published values of the maximum sodium conductance, the conductance per toxin site is estimated to be about 7 pS, assuming sequential activation and inactivation processes (F. Bezanilla & C.M. Armstrong, 1977, J. Gen. Physiol. 70:549). This single site conductance value of 7 pS is in close aggreement with estimates of the conductance of one open sodium channel from measurements of gating currents and of noise on squid giant axons and is consistent with the hypothesis that one saxitoxin molecule binds to one sodium channel.