(1) Potential differences between two microelectrodes, one in the protoplasm ofNitella and one just outside the cell have been measured. (2) The potential difference increases with increase in size of the cells studied in tap water or in .001 M NaCl from 15 m. v. for a cell volume of about 0·5 mm3 to about 40 m. v. for cells of 3 mm3 in volume. Larger cells showed irregular potential differences mostly less than 40 m. v. (3) The potential differences yielded by cells immersed in artificialNitella sap was 7 m. v.; dilution of the sap to 3/4, 1/2, or 1/4 strength led to successively greater potential differences approaching as a limit the figure for cells of similar size in tap water or .001 M NaCl. (4) Theoretical implications are discussed briefly.