A composite film, which contains multiwalled carbon nanotubes (MWCNTs) incorporated with poly(melatonin) (PMel), has been synthesized on gold electrode by the potentiodynamic method. The presence of MWCNTs in the MWCNTs-PMel composite film enhances surface coverage concentration (Gamma) of PMel to approximate to 640% and increases the electron-transfer rate constant (k(s)) to approximate to 17%. The surface morphology of the MWCNTs-PMel composite film deposited on gold electrode has been studied using scanning electron microscopy and atomic force microscopy. These two techniques reveal that PMel is incorporated in MWCNTs. Electrochemical quartz crystal microbalance study reveals the enhancement in the functional properties of MWCNTs and PMel. The MWCNTs-PMel composite film exhibits enhanced electrocatalytic activity toward the biochemical compound mixture containing 3,4-dihydroxy-L-phenylalanine (DOPA) and serotonin. The electrocatalytic responses of these analytes at PMel, MWCNTs, and MWCNTs-PMel films were measured using cyclic voltammetry. From the electrocatalysis studies, well-separated voltammetric peaks have been obtained for DOPA and serotonin in MWCNTs-PMel composite film, with a peak separation of 145.1 mV. The sensitivity of MWCNTs-PMel composite film toward DOPA and serotonin is 0.4 and 0.22 mA mM(-1) cm(-2), respectively. Similarly, the sensitivity of MWCNTs-PMel composite film toward DOPA and serotonin present in the mixture is 0.46 and 0.54 mA mM(-1) cm(-2), respectively. These sensitivity values of MWCNTs-PMel composite film are higher than the PMel and MWCNT films. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3459907] All rights reserved.