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1. The electrogenic contribution of the Na⁺-K⁺ exchange pump to the membrane potential of the Anisodoris giant neurone (G cell) was examined under steady-state and Na⁺ loaded conditions.

2. The membrane potential was variable for the first 1-4 hr after impalement, but, in the absence of experimental manipulation, remained constant thereafter. The average membrane potential for ten cells maintained at 11-13 °C and measured 5-36 hr after impalement was 55·8 ± 1·0 mV (S.E. of mean).

3. Low concentrations of external ACh caused a reversible increase in membrane Na⁺ conductance. Brief exposure to ACh proved a fast and reversible technique to load the cell with Na⁺ ions, and transiently stimulate the electrogenic Na⁺ pump.

4. In ten cells maintained from 5 to 36 hr at 11-13° C the reduction in membrane potential produced by inhibition of the Na⁺ pump with ouabain was remarkably constant between cells and averaged + 9·7 mV.

5. Cells maintained under steady-state conditions (at 11-13° C) for extended periods of time were shown to be relatively insensitive to changes in temperature and to small changes in external K⁺.

6. It is estimated that the Na⁺-K⁺ exchange pump contributes approximately - 10 mV to the steady-state resting potential of the G cell, and that two Na⁺ ions are extruded for every K⁺ ion transported into the cell per pump cycle.