The performance of an enzymatic biofuel cell in physiological media depends not only on the electrode architecture but is also influenced by substances in these media which can be directly oxidized or reduced at the anode and/or cathode or have an impact on the biocatalytic processes. For the anode construction carbon nanotubes are modified with a polyaniline film onto which pyrroloquinoline quinone dependent glucose dehydrogenase (PQQ-GDH) from Acinetobacter calcoaceticus is covalently coupled. The cathode is based on bilirubin oxidase (BOD) from Myrothecium verrucaria which is coupled to a PQQ-modified carbon nanotube electrode. The resulting biofuel cell achieves power maxima of more than 100 mu W/cm(2). Galvanodynamic performance measurements in urine and saliva show a significant loss of the maximum power density compared to an EBFC in 5 mM glucose containing buffer. The EBFC achieves 12% in urine and 18% in saliva. The initial open cell potential of about 710 mV for the EBFC is reduced to 400 mV in urine and to 665 mV in saliva. To elucidate the effect of potential interfering substances in the real media the individual electrodes are investigated by cyclic voltammetry in human urine and saliva and also in the presence of urea, uric acid and ascorbic acid. Reasons for the low power output in the both body liquids are the very low glucose concentrations. In urine the cell potential is significantly decreased because of the strong influence of an oxidation process at the cathode. The low anode performance in saliva can be attributed to the diminished biochemical catalysis. (C) 2016 Elsevier Ltd. All rights reserved.