1. The concentrations of potassium, sodium and bromide in the ciliate Spirostomum ambiguum have been measured by equilibration with radioactive media.

2. The potassium concentration in the animal is at least ten times greater than the concentration in the experimental media.

3. At the higher concentrations of the experimental media, sodium is kept out of the animal against the concentration gradient. The rate of uptake and loss of sodium is largely independent of the external concentration.

4. The time of half exchange of cell sodium is approximately 3 min., whereas for potassium it is between 2 and 3 hr.

5. After equilibration of the animals in culture medium containing bromide the cell concentration of bromide is found to be lower than the external concentration.

6. The effects of temperature changes on the uptake of potassium have been investigated. The size of the animal decreases with increased temperature but the internal potassium concentration remains fairly constant.

7. Feeding the animals increases the rate of uptake and the internal concentration of potassium.

8. It is concluded that the cytoplasm of Spirostomum is hypertonic to the medium.

9. The experimental results for cation exchange might largely be explained by exchange diffusion, but actively regulating processes seem necessary to account for the maintenance of constant internal concentrations when the animals grow and multiply.

10. It is suggested that potassium is concentrated in the cell in accordance with a Donnan equilibrium arising from the presence of indiffusible anions in the cell and an active process which keeps out sodium.

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