Light filters and metabolic inhibitors have been used to investigate further the active transport of sulphate into Chara australis. Two states of influx, light (basal) and dark (transiently stimulated), have been described. The stimulated state noted on transfer to dark has been found when the incident intensity of monochromatic light is reduced, and when photosystem 2 in photosynthesis is inhibited, either by use of cut-ofT filters or by DCMU. The light influx is insensitive to CCCP when photosynthetic 14CO2 fixation is totally inhibited, and is less sensitive to DNP than the dark influx. Dark influx is inhibited by CCCP, DNP, and NaCN but is insensitive to DCMU. It is proposed that a respiratory ATP source may be sufficient energy supply for sulphate influx and that the state of influx is under separate control. It is suggested that a ‘triggering’ mechanism may bring about the change from the light- to the dark-influx state.