A device for determining the share of the recirculation in a fistula (R A) and/or the cardiopulmonary recirculation ((R CP) in the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP) for an apparatus for extracorporeal blood treatment, the extracorporeal blood treatment apparatus comprising: an extracorporeal blood circuit, which includes a first chamber of a dialyser or filter divided by a membrane into the first chamber and a second chamber, and a fluid system which includes the second chamber of the dialyser or filter, a device for supplying blood at a specific blood flow rate into the first chamber of the dialyser or filter, a substitution device for supplying substitution fluid at a specific substitution rate to the blood upstream or downstream of the dialyser or filter and an ultrafiltration device (13, 14) for withdrawing fluid at a specific flow rate from the blood via the membrane of the dialyser or filter, characterised in that the device for determining the share of the recirculation in a fistula (R A) and/or the cardiopulmonary recirculation ((R CP) in the sum of fistula recirculation (R A) and/or cardiopulmonary recirculation ((R CP) comprises: a control unit for controlling the substitution device and the ultrafiltration device (13, 14), the control unit being designed in such a way that substitution rate Qs is changed by a predetermined amount upstream or downstream of the dialyser or filter, while flow rate Q FM of the fluid withdrawn through the membrane of the dialyser is changed, a unit for determining the blood volume (RBV(t 1)); (RBV(t 2)) or a quantity correlating with the blood volume before and after the change in the substitution rate and the flow rate of the fluid withdrawn through the membrane of the dialyser; a unit for determining the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP); and an evaluation unit, which is designed in such a way that fistula recirculation (R A) is determined from the ascertained blood volume or the quantity correlating with the blood volume before and after the change in substitution rate Q S and flow rate Q FM of the fluid withdrawn through the membrane of the dialyser or filter, and the share of the fistula recirculation (R A) and/or the cardiopulmonary recirculation ((R CP) in the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP) is determined from the ascertained fistula recirculation (R A) and the ascertained sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP). an extracorporeal blood circuit, which includes a first chamber of a dialyser or filter divided by a membrane into the first chamber and a second chamber, and a fluid system which includes the second chamber of the dialyser or filter, a device for supplying blood at a specific blood flow rate into the first chamber of the dialyser or filter, a substitution device for supplying substitution fluid at a specific substitution rate to the blood upstream or downstream of the dialyser or filter and an ultrafiltration device (13, 14) for withdrawing fluid at a specific flow rate from the blood via the membrane of the dialyser or filter, characterised in that the device for determining the share of the recirculation in a fistula (R A) and/or the cardiopulmonary recirculation ((R CP) in the sum of fistula recirculation (R A) and/or cardiopulmonary recirculation ((R CP) comprises: a control unit for controlling the substitution device and the ultrafiltration device (13, 14), the control unit being designed in such a way that substitution rate Qs is changed by a predetermined amount upstream or downstream of the dialyser or filter, while flow rate Q FM of the fluid withdrawn through the membrane of the dialyser is changed, a unit for determining the blood volume (RBV(t 1)); (RBV(t 2)) or a quantity correlating with the blood volume before and after the change in the substitution rate and the flow rate of the fluid withdrawn through the membrane of the dialyser; a unit for determining the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP); and an evaluation unit, which is designed in such a way that a control unit for controlling the substitution device and the ultrafiltration device (13, 14), the control unit being designed in such a way that substitution rate Qs is changed by a predetermined amount upstream or downstream of the dialyser or filter, while flow rate Q FM of the fluid withdrawn through the membrane of the dialyser is changed, a unit for determining the blood volume (RBV(t 1)); (RBV(t 2)) or a quantity correlating with the blood volume before and after the change in the substitution rate and the flow rate of the fluid withdrawn through the membrane of the dialyser; a unit for determining the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP); and an evaluation unit, which is designed in such a way that fistula recirculation (R A) is determined from the ascertained blood volume or the quantity correlating with the blood volume before and after the change in substitution rate Q S and flow rate Q FM of the fluid withdrawn through the membrane of the dialyser or filter, and the share of the fistula recirculation (R A) and/or the cardiopulmonary recirculation ((R CP) in the sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP) is determined from the ascertained fistula recirculation (R A) and the ascertained sum of fistula recirculation (R A) and cardiopulmonary recirculation ((R CP).