The effect of triphenyllead chloride on membrane potential and electrical conductance of internodal cells of the alga Nitellopsis obtusa was studied. It was found that the compound studied, when present at 50 μM concentration, caused membrane depolarization and increased electrical conductance. The effect is, however, observed only after 0-30 min incubation, which may be due to the cumulation of triphenyllead in the cell wall before direct contact with the plasma membrane could occur. After 30 min incubation in triphenyllead solution, the average relative changes in the membrane potential and electric conductance were -0.1±0.04 and 0.33±0.08 (n=12), respectively. These changes may be due to increased permeability of chloride ions brought about by the compound's interaction with channel proteins, or increased unspecific leakage of ions induced by a disturbance in the membrane structure.