The invention relates to a method and an apparatus for determining the percentage of fistula circulation and/or cardiopulmonary recirculation relative to the total fistula recirculation and cardiopulmonary recirculation during an extracorporeal blood treatment in which substitution liquid is fed to the blood at a specific substitution rate upstream or downstream of the first chamber (3) of the dialyzer (1) or filter, while liquid is withdrawn from the blood at a specific flow rate via the membrane (2) of the dialyzer (1) or filter. The method according to the invention and the apparatus according to the invention are characterized in that the substitution rate Qs is modified by a predefined amount upstream of downstream of the dialyzer or filter while the flow rate QFM of the liquid that is withdrawn through the dialyzer membrane is modified. The blood volume RBV(t) or a variable correlated with the blood volume, e.g. the hematocrit Hct(t), is determined before and after modifying thesubstitution rate or the flow rate of the liquid withdrawn through the dialyzer or filter membrane in order to calculate fistula recirculation RAon the basis of the blood volume or the hematocrit. Inaddition, the sum R of fistula recirculation (RA) and cardiopulmonary recirculation (RCP) is determined. The percentage of fistula recirculation (RA) and/or cardiopulmonary recirculation (RCP) relative to the total fistula recirculation (RA) and/or cardiopulmonary recirculation (RCP) is calculated from the determined fistula recirculation (RA) and the sum R of fistula recirculation (RA) and cardiopulmonary recirculation (RCP).