Tonoplast-free Chara cells whose internal ATP or Mg2+ concentrations were lowered extremely (—ATP cells, or —Mg cells) were prepared by intracellular perfusion. A rapid light-induced potential change (rLPC) across the plasma membrane was observed, when —ATP cells or —Mg cells were stained with neutral red. Kinetic analysis showed that the time courses of both the light-on and -off responses fit with simple exponential functions. The rate constant for the light-on response depends on the intensity of light. The rate constant for the light-off response was nearly constant regardless of the light intensity. The production of some chemical agent dependent on light intensity and its disappearance with darkening are indicated. The action spectrum study and the effect of 3-(3,4-dichlorophenyl)-l, l-dimethylurea on rLPC suggest that the receptor pigment is that of photosynthesis. The rLPC was observed when cells were perfused with medium containing K+ or Rb+ as the alkali metal cation, but not when the perfusion medium contained only Na+. Thus, the rLPC probably reflects an increase in K+-permeability of the plasma membrane.