1. Amoebae can be penetrated by microelectrodes at either end. One records voltage and the other supplies alternating current.
2. Step-like increases in alternating voltage superimposed on potentials recorded by the voltage electrode when in either the pseudopod or rear region demonstrate that low potentials recorded from a pseudopod and high ones from the rear region exist across a discrete impedance barrier. The only structure so far shown to fulfil this function is the plasma membrane.
3. A resistance inserted in the earth path monitors current flowing through the system and confirms observations made when recording with single electrodes that there is a reduction of electrode resistance when the cell is entered.
4. Pronounced depolarization in the rear region is shown when the current-carrying electrode penetrates the pseudopod, but not vice versa.
5. Morphological changes associated with membrane potential reversal are illustrated.
6. Consideration is given to the role of step-like potential changes in movement.