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The electrophysiological membrane parameters of the unicellular green alga Eremosphaera viridis were determined using an improved computer-supported single-microelectrode technique. These cells developed an average membrane potential of-150 mV in the light and a specific resistance of 1 Ω m(2) with an external potassium concentration of 1.1 mM and pH 5.5. In the dark, many cells showed a less polarized potential of 30-40 mV and a smaller membrane resistance. At potassium concentrations in the external medium higher than 1 mM, the membrane potential strongly depends on the external potassium content apart from a small electrogenic component. At concentrations lower than 1 mM K(+), a dependence of the membrane potential upon external potassium concentrations could not be verified. Inserting the internal ion activities in the Goldmann equation shows that, in this range, the proton conductance seems to be predominant over the potassium conductance. Transient changes in the membrane potential and in the membrane resistance were observed after switching off the light, after addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea or N,N'-dicyclohexylcarbodiimide, after a sudden decrease in temperature, and after current pulses. These changes resemble the action potentials (AP) found in other plant cells (Chara, Acetabularia). On average, the AP has a delay period of 5.1 s and a duration of 43.8 s showing a sudden decrease and a slower regeneration. The voltage peak during an AP followed exactly the Nernst potential of potassium over a range of external potassium concentrations from 5 μM to 0.2 M. This is true for depolarization or hyperpolarization, depending on the external K(+)-concentration. Tetraethylammonium-hydrogensulphate, a rather specific inhibitor of K(+) channels in nervous cells, suppressed the AP. The correlation of the appearance of the AP with a short-term opening of potassium channels in the membrane of Eremosphaera is discussed.