Presented is a flow-through method for continuous ammonium-selective enzymatic monitoring of the artificial kidney by means of a bioelectrochemical urea electrode. The urea is converted in an enzyme membrane by covalently bound urease and the ammonium ions are detected by a Nonactin-PVC-membrane. In addition to detailed data from the oxygen-independent solid-state contact sensor, curves are obtained on-line from the patient during the haemodialysis session. The advantages of the method are described in detail. Furthermore, the urea sensor can be used for measurements in heparinized blood.