Time course of the sodium permeability change during a single membrane action potential.

1. A method for measuring the time course of ionic fluxes during a non-propagated membrane action potential is described; the technique combines intracellular perfusion of the squid giant axon with radioactive tracer methods and with a method for controlling the membrane potential during small time intervals.2. The method is used to determine the temporal course of the sodium extra influx during an action potential.3. The results agree with the time course of the permeability change to sodium ions calculated with the Hodgkin & Huxley equations.4. An average extra sodium influx of 7.13 p-mole/cm(2) per action potential was determined at 12 degrees C and of 5.23 p-mole/cm(2) per action potential at 15 degrees C.