The transient depolarizations in Acetabularia which fulfill the essential criteria of an action potential (all-or-none characteristics, triggering by depolarization, propagation, etc.) are investigated. These action potentials are analyzed by conductance measurements and voltage clamp experiments on the basis of the analog circuit of the membrane (Gradmann, D. 1975, J. Membrane Biol. 25:183). It is concluded that these action potentials do not arise by permeability changes of the passive diffusion channels, but by the active pathway of the electrogenic pump, which consists of a voltage source EP of about --20 mV in series with two nonlinear conducting elements P1 and P2, the latter and EP being shunted by a large quasi capacity CP of some mF cm-2. The nonlinear current-voltage relationship of the carrier system (P1) is not changed during the action potential but has an effect on its time course. However, the elements P2 and CP, which probably reflect metabolic entities, are suggested to control the action potentials.