We report the preparation of reduced graphene oxide (RGO)/cobalt oxide (Co3O4) nanocomposite by a simple in situ electrochemical approach and its application towards the trace level selective determination of serotonin in the presence of ascorbic acid and dopamine. Simultaneous electrochemical reduction of graphene oxide (GO) to RGO and electrodeposition of Co3O4 has been achieved onto the glassy carbon electrode (GCE) surface. Raman and XRD studies confirmed the presence of RGO/Co3O4 hanocomposite formation. Scanning electron microscope (SEM) image shows that the presence of Co(3)4 nanoflakes over the entire Surface of wrinkled thin graphene nanosheets. The electro-oxidation of serotonin has been assessed by cyclic Voltammetry and differential pulse voltammetry (DPV) in physiological pH conditions. Interestingly, the oxidation peak current (I-p,) of serotonin at GCE/RGO/Co3O4 is 4.1 fold higher than that observed at GCE/RGO and 2.4 fold higher than that observed at GCE/Co3O4 modified electrode. A DPV serotonin sensor with a very low detection limit of 48.7 nM and a high sensitivity of 2.2 mA mM(-1) cm(-2) has been demonstrated Using the as prepared GCE/RGO/Co3O4 modified electrode. A comparison of the sensor performance with previous literature clearly shows the superior performance of the fabricated sensor. In addition, the fabricated sensors detect serotonin in vitro directly from diluted blood serum samples. (C) 2017 Elsevier B.V. All rights reserved.