An apparatus and method of determining the blood flow rate at which, in an extra-corporeal blood circuit of an extra-corporeal blood treatment apparatus, blood is pumped through the arterial blood line into the blood chamber of the dialyser and out of its blood chamber through the venous blood line is described. The method and apparatus provide for the determination of a plurality of parameters characteristic of the extra-corporeal blood treatment, with a given blood flow rate being determined in each case as a function of one of the characteristic parameters. A blood flow rate which is preset for the blood treatment is selected from the plurality of blood flow rates determined on the basis of the characteristic parameters. The blood flow rate to be preset is selected on the basis of a preset algorithm which may be implemented in software installed on a data processing unit or in hardware.
