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1. The membrane current caused by the iontophoretic injection of sodium into giant neurones of the snail Helix pomatia was investigated under a long lasting voltage clamp. The inhibition of this current by ouabain (10^-4 M) and by cooling to + 7° C confirmed its link with the active transport of ions. Therefore this current is called the pump current.

2. Over the range of membrane potential -40 to -100 mV the changes in the steady current—voltage curves caused by the pump current development were investigated. The pump current was found to be potential-dependent. It decreased with increasing hyperpolarization of the neurone.

3. With large hyperpolarizations the current—voltage curves obtained before the sodium injection and after eliciting the pump current coincided with each other. An increase in the membrane conductance was observed over the range of membrane potential corresponding to the pump current display.

4. The applied sodium injections did not cause any marked changes in the passive permeability of the membrane. This fact made it possible to measure the charge transferred across the membrane during operation of the pump current. Unlike previous data, the ratio of this value to the charge used to inject sodium into the neurone appeared to be a variable.

5. When the preparation was cooled to + 11° C, and also during the first few minutes after the application of a potassium-free solution, both the pump current and the membrane potential at which it disappeared could increase.

6. The pump current measurements during a number of transitions from one fixed level of the membrane potential to another showed that the current did not depend upon the potential at which it developed before each transition.

7. The data presented allow the suggestion that the potential dependence of the pump current is determined by the changes in the rate of active transport of potassium, while the rate of active transport of sodium remains constant.