Abstract: The present invention is directed to a method and an apparatus for continuously monitoring the concentration of a metabolite, such as glucose or lactic acid, in biological tissue, in which a perfusion fluid is fed to a microdialysis probe implanted in the subcutaneous tissue and removed therefrom as dialysate after enrichment with the metabolites contained in the lymph. An enzyme is added to the dialysate and/or perfusion fluid, and the concentration of the metabolite in the dialysate is determined under the selective effect of the enzyme at a measuring point which is positioned ex vivo, using an electrochemical sensor. The enzyme is added to the dialysate in the form of a continuous enzyme solution flow upstream of the measuring point and in dependence of the throughput of the microdialysis probe.