The membrane potential ofStentor coeruleus was measured by means of glass microelectrodes. The tips of the electrodes had a diameter of 2 μ, and were filled with 0.1 M KCl. The maximum value of the potential is −30.8±8.6 mV and is reached within 600–800 msec after electrode insertion; the potential then decreases to a negative value of a few mV within 5–10 sec. Contraction initiated by mechanical stimulation or by rapid cooling is correlated with a positive potential jump of about 3–5 mV. Similarly, a fully reversible potential shift develops in close correlation to the reversal of ciliary beat in oral membranelles.