A device for extracorporeal blood treatment, in particular for hemodialysis, comprising: a dialyzer rendering possible an exchange of substances between a patient's blood and a dialysis fluid, a proportioning unit, which provides fresh dialysis fluid and is connected to the dialyzer via a supply line for supplying fresh dialysis fluid, the dialyzer further being connected to a discharge line for discharging used dialysis fluid from the dialyzer, a first measuring apparatus provided in the supply line for detecting the conductivity of the fresh dialysis fluid, a second measuring apparatus, provided in the discharge line, for detecting the conductivity of the used dialysis fluid, a control unit connected to the proportioning unit and the measuring apparatuses (8, 12) for data exchange, and wherein a third measuring apparatus is provided in the discharge line for measuring or estimating a loading of the used dialysis fluid with urophanic substances, and the third measuring apparatus is connected to the control unit for data exchange, characterized in that the control unit is designed to continuously control in open-loop or closed-loop fashion, on the basis of the conductivities detected continuously during blood treatment by the first measuring apparatus and the second measuring apparatus, and the loading with urophanic substances detected or estimated by the third measuring apparatus, the concentration of a substance in the fresh dialysis fluid provided by the proportioning unit during the blood treatment in such a way that it is equal to the concentration of the substance in the used dialysis fluid or is greater or less than the concentration of the substance in the used dialysis fluid by a predetermined value. a dialyzer rendering possible an exchange of substances between a patient's blood and a dialysis fluid, a proportioning unit, which provides fresh dialysis fluid and is connected to the dialyzer via a supply line for supplying fresh dialysis fluid, the dialyzer further being connected to a discharge line for discharging used dialysis fluid from the dialyzer, a first measuring apparatus provided in the supply line for detecting the conductivity of the fresh dialysis fluid, a second measuring apparatus, provided in the discharge line, for detecting the conductivity of the used dialysis fluid, a control unit connected to the proportioning unit and the measuring apparatuses (8, 12) for data exchange, and wherein a third measuring apparatus is provided in the discharge line for measuring or estimating a loading of the used dialysis fluid with urophanic substances, and the third measuring apparatus is connected to the control unit for data exchange, characterized in that the control unit is designed to continuously control in open-loop or closed-loop fashion, on the basis of the conductivities detected continuously during blood treatment by the first measuring apparatus and the second measuring apparatus, and the loading with urophanic substances detected or estimated by the third measuring apparatus, the concentration of a substance in the fresh dialysis fluid provided by the proportioning unit during the blood treatment in such a way that it is equal to the concentration of the substance in the used dialysis fluid or is greater or less than the concentration of the substance in the used dialysis fluid by a predetermined value.