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1. Binding of tritiated ouabain to the smooth muscle of the guinea-pig taenia coli showed two components, one saturable at lower glycoside concentrations and the other linear with increasing concentrations.

2. The saturable component alone was affected by extracellular potassium concentrations. This component seems to be bound to sodium pumping sites, and when completely saturated binds 1·1 x 10¹¹ molecules per mg fresh wt. of tissue, or 250-300 molecules per square micron of membrane, assuming a volume:surface area ratio of 1·5 µm.

3. Only a fraction of ⁴²K uptake by the cells can be blocked by ouabain at maximal concentrations. In normal Krebs solution two thirds can be blocked. The remaining one third is presumably passive uptake. The fraction blocked is reduced as the extracellular potassium concentration, and thus passive uptake, is increased.

4. The amount of potassium pumped into the cells at various concentrations of extracellular potassium has been calculated. In normal Krebs solution the amount pumped in 45 min was 20·0 m-mole/kg fresh wt., and this was increased at higher potassium concentrations.

5. On the assumption of a stoichiometry of 3Na: 2K, the pump sites in normal Krebs solution have a turnover rate of 1320 min^-1.

6. Indirect calculations of sodium movements suggest that the sodium permeability may be about 0·9 x 10^-8 cm sec^-1 and the pump may generate a current of 0·9 x 10^-7 A cm^-2. This crossing an Ohmic membrane resistance of 30-60 k cm² would be equivalent to a potential difference of 3-5 mV.