NOVELTY - The method for electrochemical measurement of an analyte concentration in vivo having a fuel cell with which the analyte to be measured is reacted catalytically with an enzyme contained in an enzyme layer (3) and which supplies an electrical voltage, dependent on the analyte concentration to be measured, between an anode (1) and a cathode (2), which voltage is measured, comprises producing a product in the catalytic reaction of the analyte to be measured in the enzyme layer, where the product is reduced as fuel of the fuel cell oxidizes on the anode and the cathode.    USE - The method is useful for electrochemical measurement of an analyte concentration in vivo having a fuel cell, which is useful for measuring an analyte in the body of a patient and an energy source of an implanted device such as a cardiac pacemaker.    ADVANTAGE - The method ensures easy and economical electrochemical measurement of an analyte concentration with improved durability and accuracy.    DETAILED DESCRIPTION - The method for electrochemical measurement of an analyte concentration in vivo having a fuel cell with which the analyte to be measured is reacted catalytically with an enzyme contained in an enzyme layer (3) and which supplies an electrical voltage, dependent on the analyte concentration to be measured, between an anode (1) and a cathode (2), which voltage is measured, comprises producing a product in the catalytic reaction of the analyte to be measured in the enzyme layer, where the product is reduced as fuel of the fuel cell oxidizes on the anode and the cathode. The oxidation of the product is supported with a catalyst such as a metal oxide or a metal organic compound. The anode and the cathode are connected together over an electrical resistance and the electrical voltage is measured as measuring signal of the analyte concentration between the anode and the cathode as voltage drop across the resistance. INDEPENDENT CLAIMS are included for:    (1) a fuel cell for electrochemical measurement of an analyte concentration in vivo; and    (2) an electrochemical sensor for in vivo measuring an analyte concentration by a fuel cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a sensor.    Anode (1)    Cathode (2)    Enzyme layer (3)    Carrier (4)    Contact surfaces. (5a, 6a)