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The sodium efflux from perfused squid giant axons has been studied using radioactive sodium, and the sufficient conditions for the maintenance of a potassium- and ouabain-sensitive sodium efflux have been established. The following were found.

1. Axons extruded and then perfused with their own axoplasm had a sodium efflux which was sensitive to cyanide, potassium and ouabain and was thus similar to the efflux from intact axons.

2. A method for replacing natural axoplasm into fibres previously perfused with artificial axoplasm was developed and used to establish an artificial perfusate that was not irreversibly toxic.

3. Short perfusion (5 min) with a variety of artificial perfusates was then found to give fibres which had potassium- and ouabain-sensitive sodium effluxes when ATP was present in the perfusate.

4. In the absence of ATP the sodium efflux was small and relatively insensitive to both external potassium and to ouabain.

5. With ADP in the perfusate, fibres gave a sodium efflux which was ouabain-sensitive but was little affected by the removal of external potassium from the sodium-rich sea water bathing the fibres.

6. The perfused fibres differed from intact fibres in having large ouabain-insensitive sodium effluxes.

7. After very long perfusions (40-90 min), with the simple media containing ATP, the rate constant for sodium efflux from the fibres tended to be large and was relatively insensitive to potassium or to ouabain.

8. Fibres refilled with natural axoplasm after long perfusion showed increased sensitivity to external potassium; refilled with dispersed axoplasm the sodium efflux tended to become very large.

9. After very long perfusions with artificial axoplasms containing ATP, a potassium- and ouabain-sensitive sodium efflux was found to persist provided that dextran was present and the total osmotic pressure and the hydrostatic pressure of the perfusate were controlled. Under these conditions the sodium efflux resembled that from briefly perfused fibres.

The necessary and sufficient conditions for the maintenance of sodium transport by perfused giant axons are discussed.