An apparatus for controlling the concentration of e.g. an anaesthetic in gas mixture in an anaesthetic system (1) comprises a gas-concentration meter (8) for measuring the concentration of the anaesthetic in the gas mixture before the point (30) at which the anaesthetic is supplied to the gas mixture, a flow meter (9) for measuring the flow rate of the gas mixture, and a dosing unit (10) for supplying the anaesthetic to the gas mixture. The gas-concentration meter (8) and the dosing point (30) are provided in the inhalation branch (2) of the anaesthetic system (1). A control unit (12, 13) determines, with the aid of the flow rate of the gas mixture in the inhalation branch (2) and the measured concentration of anaesthetic in the gas mixture, the flow rate of the anaesthetic to be supplied to the gas mixture to give the desired concentration of anaesthetic. The apparatus and the method can also be used for controlling the concentration of other components than an anaesthetic in a gas mixture.