The high membrane potential of Acetabularia (Em=-170mV) is due to an electrogenic pump in parallel with the passive diffusion system (Ed=-80mV) which could be studied separately in the cold, when the pump is blocked. Electrical measurements under normal conditions show that the pump pathway consists of its electromotive force Ep with two elements P1 and P2 in series; P2 is shunted by a large capacitance (Cp=3mF cm-2). The nonlinear current-voltage relationship of P1 (light- and temperature-sensitive) could be determined separately; it reflects the properties of a carrier-mediated electrogenic pump. The value of Ep(-190 mV) indicates a stoichiometry of 2:1 between electrogenically transported charges and ATP. The electrical energy normally stored in Cp, compares well with the metabolic energy, stored in the ATP pool. The nonlinear current-voltage relationship of P2 (attributed to phosphorylating reactions) is also sensitive to light and temperature and is responsible for the region of negative conductance of the overall current-voltage relationship. The power of the pump (1 muW cm-2) amounts to some percent of the total energy turnover. The high Cl- fluxes (1 nmol cm-2 sec-1) and the electrical properties of the plasmalemma are not as closely related as assumed previously. For kinetic reasons, a direct and specific Cl- pathway between the vacuole and outside is postulated to exist.