A new rapid, convenient and sensitive electrochemical method based on a Nafion membrane-coated colloidal gold screen-printed electrode (Nafion/CGSPE) was described for the determination of serotonin in the presence of high concentration of dopamine, ascorbic acid and uric acid. The electrocatalytic response of serotonin on the Nafion/CGSPE was evaluated by cyclic voltammetry (CV) in a 0.1 M phosphate buffer (pH 7.4) and the modified electrode exhibited better selectivity and higher sensitivity than the bare SPE. The linear calibration Curve was obtained in the range of 2 x 10(-8)-5 x 10(-4) M, with a correlation coefficient of 0.9969, and a detection limit of 5.0 nM in differential Pulse voltammetry (DPV). We obtained linearity with correlation coefficient of 0.9971 for adding 0-4 x 10(-6) M serotonin in the brain samples and 0.9978 in the blood samples. The adequacy of the developed method was successfully applied to the determination serotonin in the platelet-rich plasma (PRP) and in the brain homogenate of the mice. The correlation between the concentration of serotonin in the blood and brain of the depressed mice was studied. (C) 2009 Elsevier B.V. All rights reserved.