Typical spontaneous action potentials can be elicited in 10–100 mM NaCl or LiCl solution. The period of repetition is 0.5–2 seconds and the action potential generally consists of a rapid spike alone. Similar spontaneous action potentials are also demonstrated by adding either 1 mM EDTA (pH 6.6) or 2 mM ATP (pH 6.6) to the artificial pond water. In these cases, however, the period of repetition is much longer and the action potential is of a normal shape, a rapid spike being followed by a slow transient depolarization. The period of repetition and the size of the action potential decrease with the elevation of the vacuolar potential level. The cause of the spontaneous firing is supposed to be the removal of Ca++ from the outer surface of the Nitella membrane.