An artificial kidney comprising a filter which is disposed between an artery connection and a vein connection, a collecting means for the filtrate to be discharged from the filter, a means for providing a metered supply of substituate to the purified blood, and a measuring means including two weighing devices; a first of the weighing devices measures the weight of filtrate to be discharged, and a second of the weighing device measures the weight of the substituate; the measuring means controls the providing means whereby the metered supply of substituate is in a fixed weight proportionality to the filtrate weight.
