A controller configured to control a blood treatment device for controlling a filtration rate during treatment of a patient's blood by means of said blood treatment device, the controller comprising: - a target relation defining means, wherein the target relation defining means is configured for defining before or during blood treatment: a target relation (7, 103, 105) during blood treatment between a first parameter, the first parameter reflecting the mass or the concentration of a substance comprised by a tissue or a bodily fluid of the patient; and a second parameter, the second parameter reflecting a distribution space of the patient or an approximation of the distribution space of the patient, wherein the distribution space of the patient is a volume of blood or another bodily fluid of the patient, or a ratio of such volumes; or a development of said target relation (7, 103, 105) during blood treatment; - a calculation means, wherein the calculation means is configured for, during the blood treatment repeatedly: calculating first value(s), the first value(s) being value(s) of the first parameter during the blood treatment; calculating second value(s), the second value(s) being value(s) of the second parameter during the blood treatment; and determining the relation between one of the first value(s) and one of the second value(s); and - a signal output means programmed for outputting one or more signals based on the defined target relation (7, 103, 105) and on the determined relation to a controlling means configured for controlling the filtration rate of the fluid treatment device in the intention that, in the course of the blood treatment, the determined relation is or approaches the defined target relation (7, 103, 105). - a target relation defining means, wherein the target relation defining means is configured for defining before or during blood treatment: a target relation (7, 103, 105) during blood treatment between a first parameter, the first parameter reflecting the mass or the concentration of a substance comprised by a tissue or a bodily fluid of the patient; and a second parameter, the second parameter reflecting a distribution space of the patient or an approximation of the distribution space of the patient, wherein the distribution space of the patient is a volume of blood or another bodily fluid of the patient, or a ratio of such volumes; or a development of said target relation (7, 103, 105) during blood treatment; a target relation (7, 103, 105) during blood treatment between a first parameter, the first parameter reflecting the mass or the concentration of a substance comprised by a tissue or a bodily fluid of the patient; and a second parameter, the second parameter reflecting a distribution space of the patient or an approximation of the distribution space of the patient, wherein the distribution space of the patient is a volume of blood or another bodily fluid of the patient, or a ratio of such volumes; or a first parameter, the first parameter reflecting the mass or the concentration of a substance comprised by a tissue or a bodily fluid of the patient; and a second parameter, the second parameter reflecting a distribution space of the patient or an approximation of the distribution space of the patient, wherein the distribution space of the patient is a volume of blood or another bodily fluid of the patient, or a ratio of such volumes; or a development of said target relation (7, 103, 105) during blood treatment; - a calculation means, wherein the calculation means is configured for, during the blood treatment repeatedly: calculating first value(s), the first value(s) being value(s) of the first parameter during the blood treatment; calculating second value(s), the second value(s) being value(s) of the second parameter during the blood treatment; and determining the relation between one of the first value(s) and one of the second value(s); and calculating first value(s), the first value(s) being value(s) of the first parameter during the blood treatment; calculating second value(s), the second value(s) being value(s) of the second parameter during the blood treatment; and determining the relation between one of the first value(s) and one of the second value(s); and - a signal output means programmed for outputting one or more signals based on the defined target relation (7, 103, 105) and on the determined relation to a controlling means configured for controlling the filtration rate of the fluid treatment device in the intention that, in the course of the blood treatment, the determined relation is or approaches the defined target relation (7, 103, 105).