The invention concerns a system for monitoring the concentration of analytes in body fluids, in particular in interstitial fluid and comprises a catheter having an implantable region and an outlet opening for withdrawing fluid in particular body fluid. A first and a second analytical zone are contacted sequentially with fluid from the catheter and undergo a detectable change when an analyte is present. The analytical zones can be contacted manually with fluid and also preferably in an automated fashion by means of a device. A system according to the invention additionally has an analytical device for analysing the analytical zones in order to determine the concentration of the analyte on the basis of changes caused by the analyte. A further subject matter of the present invention are catheters for use in systems according to the invention as well as magazines containing test zones.