Irrtraoellular microelectrodes have been used to record the action potential in the vacuole and cytoplasm of single cells of the alga N itella. The N itella cells were cultured in White's nutrient solution (9�2 X 10-3 g total salta/I) and almost all measurements of the action potential were made on cells in this solution. The resting potentials of the vacuole and cytoplasm with respect to the bathing solution are approximately equal, and have a value of -150 mV.