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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.

Isolated axons from the squid, Dosidicus gigas, were internally perfused with potassium fluoride solutions. Membrane currents were measured following step changes of membrane potential in a voltage-clamp arrangement with external isosmotic solution changes in the order: potassium-free artificial seawater; potassium chloride; potassium chloride containing 10, 25, 40 or 50, mM calcium or magnesium; and potassium-free artificial seawater. The following results suggest that the currents measured under voltage clamp with potassium outside and inside can be separated into two components and that one of them, the predominant one, is carried through the potassium system. (a) Outward currents in isosmotic potassium were strongly and reversibly reduced by tetraethylammonium chloride. (b) Without calcium or magnesium a progressive increase in the nontime-dependent component of the currents (leakage) occurred. (c) The restoration of calcium or magnesium within 15-30 min decreases this leakage. (d) With 50 mM divalent ions the steady-state current-voltage curve was nonlinear with negative resistance as observed in intact axons in isosmotic potassium. (e) The time-dependent components of the membrane currents were not clearly affected by calcium or magnesium. These results show a strong dependence of the leakage currents on external calcium or magnesium concentration but provide no support for the involvement of calcium or magnesium in the kinetics of the potassium system.