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1. When red cells loaded with ²⁴Na are incubated in balanced glucose salt solutions, the removal of external potassium reduces sodium efflux by about one third. Ouabain reduces the residual sodium efflux by about one half.

2. The ouabain-sensitive efflux of sodium into potassium-free solutions is accompanied by an equal ouabain-sensitive influx of sodium.

3. If sodium in the external potassium-free salt solution is progressively replaced with choline, both the ouabain-sensitive sodium efflux and the ouabain-sensitive sodium influx are reduced until, with only 5 mM-Na externally, both become very small.

4. At sodium concentrations intermediate between 140 mM and 5 mM, ouabain-sensitive sodium influx and ouabain-sensitive sodium efflux remain equal within the limits of experimental error. The relation between the magnitude of the ouabain-sensitive sodium exchange and the external sodium concentration is roughly linear.

5. As the external sodium concentration is decreased beyond 5 mM towards zero, ouabain-sensitive sodium efflux increases again.

6. In the presence of 5 mM-K, the ouabain-sensitive sodium efflux is scarcely affected by replacing most of the external sodium with choline.

7. In the presence of ouabain, sodium efflux is unaffected by external potassium and is little affected by replacing external sodium with choline.

8. The results suggest that in the absence of external potassium the ouabain-sensitive transport mechanism catalyses a one-for-one exchange of sodium ions across the cell membrane. The relation between this exchange and Ussing's classical `exchange diffusion' is discussed.

9. The exchange does not occur in the presence of external potassium at physiological concentrations.

10. Calculations of sodium efflux were based on the specific activities of total cell sodium: separate experiments confirmed that these were the same as the specific activities of sodium lost to sodium-free solutions. No evidence of a slowly exchanging sodium fraction was found.

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