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1. A study has been made of the effect of potassium and other cations on the oxygen consumption of rabbit desheathed vagus nerves at rest and during activity.

2. In normal Locke solution (containing 5·6 mM-K) the resting oxygen consumption, Q_r, was 0·0903 µmole/g wet wt. min. The extra oxygen consumed as a result of stimulation, Q_s, at 3 stimuli/sec was about 600 p-mole/g.impulse; at 30 stimuli/sec it was less, being 240 p-mole/g.impulse.

3. Only a fraction of Q_r (18% at 5·6 mM-K) was sensitive to ouabain (1 mM). The ouabain-sensitive component, however, increased as the external potassium concentration was increased, in the range 0-100 mM. Q_s was virtually abolished by ouabain.

4. Reduction of the external potassium concentration from 5·6 mM to zero reduced Q_r (by 10%) and increased Q_s, but the changes were scarcely significant statistically.

5. The conclusions were drawn: that Q_s reflected the pumping of cations to restore the ionic imbalance following activity, particularly reflecting the extrusion of sodium ions from the fibre; that this pumping was normally absolutely dependent on the presence of potassium externally and that no pumping could occur in its absence; and that Q_s was not reduced to zero in ostensibly potassium-free solutions because enough potassium was released into the periaxonal space during activity to maintain pumping.

6. Thallium, rubidium, caesium and lithium could replace potassium and allowed pumping to occur.