A poly(safranine O) modified glassy carbon electrode has been prepared by an electrooxidative polymerization of safranine O. The electrochemical behavior of serotonin (5-HT) at this modified electrode was investigated by cyclic voltammetry (CV) and square wave adsorptive stripping voltammetry. The modified electrode was employed as an electrochemical sensor for the detection of serotonin concentration and exhibited a typical enhance effect on the current response of serotonin and lower oxidation overpotential. Due to the relatively low currents and different potentials in the electrochemical responses to ascorbic acid and dopamine, the modified electrode is a useful and effective sensing device for the sensitive and selective serotonin determination in the presence of ascorbic acid and dopamine. At optimised conditions, the calibration curve for serotonin was showed two linear segments: the first linear segment increased from 3.0 x 10(-8) to 1.0 x 10(-6) mol L-1 and second linear segment increased up to 1.0 x 10(-5) mol L-1. The detection limit was determined as 5.0 x10(-9) mol L-1 using SW-AdSV. The proposed method was successfully applied to determine serotonin in human serum samples.