Abstract: For a potentiostatically operable, polarographic-amperometric sensor, in particular for biomedical use, wherein a working electrode (5) and a reference electrode (4) are covered by an electrolyte layer (9) which is separated from the medium to be measured by a semipermeable membrane (8), the invention proposes to provide the counter electrode (6) in the form of an external electrode which--for example--can be an outer metal coating surrounding the sensor chip (2) in a three electrode system fabricated as micro-sensor with the integrated technique, and which can be made--for example--of a stainless steel material or of a conductive plastics material.