In the present study, the electrocatalytic oxidation of reduced beta nicotinamide adenine dinucleotide (NADH) was investigated using a pencil graphite electrode modified with quercetin (PGE/QH(2)). The PGE/QH(2) was prepared through two steps: (i) the pre-treatment of PGE at 1.40V vs. Ag vertical bar AgCl vertical bar KCl(sat.) in pH 7.0 phosphate buffer containing 0.1 M KCl for 60 s and (ii) adsorption of QH(2) on the PGE via immersion of PGE into a 1.0 mM QH(2) solution (in ethanol) for 60 s. Cyclic voltammetric studies show that the peak potential of NADH oxidation shifts from +500 mV at bare PGE to +300 mV at PGE/QH(2). The electrocatalytic currents obtained from amperometric measurements at +300 mV vs. Ag vertical bar AgCl vertical bar KCl(sat.) and in phosphate buffer solution at pH 7.0 containing 0.1 M KCl were linearly related to the concentration of NADH. Linear calibration plots are obtained in the concentration range from 0.5 mu M to 100 mu M. The limit of detection was found to be 0.15 mu M. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.