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1. A study has been made of the ionic content, and of the fibre water, of rabbit desheathed vagus nerves at rest and after activity in various modified Locke solutions.

2. In normal Locke solution the intracellular sodium amounted to 86·1 µmole/g dry, and the intracellular potassium 186·0 µmole/g dry. Since the fibre water amounted to 1·124 g/g dry the intracellular sodium and potassium concentrations were 76·6 and 165·5 m-mole/kg fibre water, respectively.

3. In potassium-free Locke solution the intracellular sodium and potassium concentrations were 111·0 and 121·7 m-mole/kg, respectively. The intracellular chloride concentration was 40·5 m-mole/kg.

4. In normal Locke solution, a brief period of stimulation followed by a 10 min recovery period produced no significant change in the intracellular contents of potassium, sodium, or water.

5. However, in chloride—Locke solution from which the potassium had been omitted, or to which ouabain (1 mM) had been added, stimulation caused a loss of potassium per impulse of about 20 p-mole/mg dry, i.e. about 4 p-mole/mg wet. A similar loss was found in isethionate—Locke solution that was potassium-free. There was no significant change in any of the other quantities measured (sodium, chloride, and fibre water).

6. The efflux of radioactively labelled potassium was measured in potassium-free Locke solution at rest (k_r) and during activity (k_s). At 24° C, k_r was 0·0084 min^-1 and k_s was 0·000178 impulse^-1.

7. When the chloride of Locke solution was replaced by isethionate there was an increase in both k_r (about 12%) and k_s (about 21%).

8. Ouabain (1 mM) increased k_s (by about 70%). This increase seemed to be independent of whether the anion present was chloride or isethionate.

9. It is argued that these results indicate that the electrogenicity of the pump plays a relatively small role in maintaining the ionic balance in mammalina C fibres.