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1. The rate constant for Na efflux from the oocyte calculated from (d/dt) (ln [Na^*]_i]) is only approximately 52% of that calculated from (d/dt)[(ln(d[Na^*]_i)dt)]. The difference may be interpreted by supposing that 48% of the internal Na of the oocyte is either bound to proteins or sequestered in cell organelles.

2. The mean rate constant for Na efflux was 6·4 x 10^-3 min^-1 corresponding to an apparent Na efflux rate of 13·3 p-mole/cm².sec. When this is corrected for the increase in surface area produced by microvilli the true efflux rate is 1·1-1·3 p-mole/cm².sec.

3. The action of ouabain (1-5 µM) appears to involve two different effects: (a) there is 48-65% inhibition of the membrane Na pump, and (b) there is a release of some of the sequestered Na in the cell.

4. Removal of external K causes a 40% reduction in Na efflux although this value may be an underestimation owing to the presence of K which has leaked from the cell and may be retained near the cell surface.

5. Raising the external K concentration to 15 mM reduces the inhibitory effect of ouabain by approximately a half.

6. It was concluded that the Na pump in the toad oocyte may have a slightly lower level of activity than that in frog muscle, but that its general properties are similar to those in frog muscle and some other animal cells.