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1. A study has been made of the binding of radioactively labelled ouabain by desheathed rabbit vagus nerves, which consist mainly of non-myelinated fibres. The corresponding inhibition of the electrogenic sodium pump was also measured.

2. By varying the ouabain concentration and the external potassium concentration two kinds of binding sites could be distinguished: a first site specifically associated with pumping and whose ability to bind ouabain is dependent on the external presence of potassium; and a second site not associated with pumping and unaffected by external potassium.

3. Just complete inhibition of the sodium pumping mechanism is associated with a specific binding of ouabain of about 4·3 p-mole/mg dry nerve.

4. This gives an upper limit for the density of sodium pumping sites of about 750 per square micron.

5. The turnover rate (i.e. (cation pumped)/(number of sites)) at 20° C is about 22 sec^-1.