;When an internodal cell of Chara corallina was stimulated with a mechanical pulse of various amplitudes lasting for 0.1 s (mechanical stimulus), the cell generated a receptor potential, which was highly dependent not only on the strength of the stimulus but also on the extracellular Cl -concentration. Extracellular Ca 2+ was indispensable for generating receptor potential, since removal of Ca 2+ reversibly inhibited generation of the receptor potential. The cytoplasmic Ca 2+ level transiently rose upon mechanical stimulation. The stronger the mechanical stimulus, the larger was the increase in the cytoplasmic level of Ca 2+ . It is proposed that the first step of receptor potential is an activation of mechanosensitive Ca 2+ channels at the plasma membrane.