Abstract: A toner replenishment control apparatus does not require waiting between replenishment cycles for a period sufficient to insure that the toner in the station is well mixed and charged, and yet does not require artificially limiting the amount of toner added to avoid over-concentration. Information is stored concerning the rates of toner addition in response to a detected concentration error, and the stored information is used in conjunction with future measurements of conconetration error to determine the correct amount of toner needed. More specifically, the replenishment rate is based on (1) the present toner concentration error, (2) an initial replenishment rate which occurred sufficiently prior to insure that the added toner is well mixed and charged, and (3) the toner concentration error which dictated that initial replenishment rate.