The urea content monitoring during hemodialysis enables the feed-back within the system "patient--artificial kidney--patient". The existing methods of determining the concentration of urea in the dialyzing solution require an expendable reagent, i.e. urease, they are discrete and need often a calibration. The electrochemical urea analyzer, worked out by the authors, is easy in use, it provides a continues information about the urea concentration and does not virtually need any calibration. Besides, the plotter, belonging to the set, provides the graphic information about the dynamic changes of urea content.