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1. Comparisons were made between the kinetics and steady-state properties of the sodium conductance changes and of the sodium gating currents, in the squid giant axon perfused with caesium fluoride and maintained at a high membrane holding potential. The conductance measurements were made with reduced external sodium and as much electrical compensation as possible, in order to minimize errors caused by the series resistance. 2. After an initial delay of 10-150 musec whose size was a function of the holding potential and pulse amplitude, the conductance rise on depolarization followed cube law kinetics. 3. Values of the time constant taum, as defined by Hodgkin & Huxley (1952b), were determined for membrane potentials ranging between -140 and +70 mV. They lay on a nearly symmetrical bell-shaped curve with maximum (at 6-3 degrees C) of just under 500 musec at -36 mV. 4. Values of the gating current time constant tau(V) were determined over the same potential range, and found to lie on a very similar bell-shaped curve. A computed least-squares best fit gave the maximum as 460 musec, also falling at about -36 mV. 5. The midpoint of the minfinity curve lay at -34 mV, and its slope at this point was 0-0139 mV-1. Another series of measurements on intact axons gave a midpotential of -25 mV. In the perfused axons the state of the membrane was better described by the constant field equation than by gNa. Recalculation of minfinity from PNa shifted the curve about 15 mV in a positive direction.

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