The conductance of sodium channels under conditions of reduced current at the node of Ranvier.

1. The single sodium channel conductance gamma and the number of channels N were estimated from fluctuation analysis in voltage-clamped nodes of Ranvier under conditions that decreased the size of the sodium current. 2. Reduction of the sodium current by depolarizing prepulses had no effect on gamma, and, in cases where it could be determined, had no significant effect on N. Partial block of the sodium conductance with tetrodotoxin and saxitoxin also did not affect gamma significantly, but reduced N. 3. gamma was reduced to about 40% of the control value at -5 mV when the pH of the external solution was reduced to 5.0. The pH dependence of gamma is consistent with the theories of Woodhull and of Drouin & Neumcke. 4. The differing effects of prepulses, toxins and pH are interpreted in view of the different time scales of channel inactivation or block under these conditions. 5. The nearly unchanged gamma with prepulses and partial toxin block provides further evidence for the absence of interactions among sodium channels.