The effect of common inorganic ions and of plant-growth-regulating substances on the cell membrane of living Nitella is examined by introducing a micro-silver/silver chloride electrode into the cell vacuole and measuring the electrical impedance of the membrane under conditions of equilibrium. When simple electrolytes are added to the external solution the resistance-concentration curves show two maxima. The first is always the same but the second different for various cations. It is suggested that the protoplasm/external solution interface lies within the interstices of the cell wall, and its total area is therefore only a fraction of the total cell-surface area. With the addition of simple electrolytes, changes in the area of protoplasmic surface exposed to the external solution are reflected by changes in impedance. The first peak in the resistance-concentration curves is interpreted as an electro-capillary curve. The second may be attributed to modification of the electric double layer at the interface followed by movement of the latter caused by the ever-increasing osmotic pressure of the external solution. Plasmolysis occurs at isotonic concentrations of electrolytes examined. Plant-growth substances cause a profound increase in the membrane resistance but do not affect the capacity. Only their undissociated forms are active, and in some cases the presence of calcium ions is necessary.