A measurement method for determining the blood flow rate QF in blood carrying lines is provided. It may be used in particular to determine the blood flow in a patient's vessel, which is connected to the extracorporeal circulation of a blood treatment machine by an arterial line and a venous line. According to the method, the net rate dX/dt of a variable X is determined, with X being derived from a physicochemical variable Y of the blood with the help of values YA and YV which are adequately constant over time and which respectively characterize the physicochemical property in the arterial line and the venous line during the measurement interval. The net rate dX/dt is then used to determine the blood flow rate QF. The targeted use of indicators is not necessary.