1. The electrical potential which exists between the cells interior and the outer surface of Paramecium when these are connected by a solution, depends largely on the nature of this solution.
2. The effect of the external pH (6·0-8·0) seems to be negligible.
3. The nature of the cation largely determines the potential observed. The effect of the anion is less marked.
4. In all solutions tested, the more dilute the solution, the more negative is the charge on the cell interior. Hence it seems probable that the membrane of Paramecium is more permeable to cations than to anions.
5. On mixing two solutions of equivalent normality, the potential observed in the mixture is intermediate in value between those measured in the solutions before mixing and the deviation is proportional to the ratio of the volumes mixed. Certain electrolytes (LaCl3 or CeCl3 are anomalous in this respect.
6. It is shown that in wiring a Lindemann electrometer it is important to insert a reversing key between the paired electrodes and the electrometer, when using high-resistance electrodes.