Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A rapid and simple capillary electrophoresis (CE) method was developed to determine 5-hydroxytryptophan (5-HTP) in samples of commercial dietary supplements. The optimized background electrolyte (BGE) was composed by 20 mmol L-1 sodium phosphate (pH 10) and 0.2 mmol L-1 cetyltrimethylammonium bromide (CTAB) for electroosmotic flow inversion. The limits of detection (LOD) and quantification (LOQ) for 5-HTP were 3.1 and 10.1 mu mol L-1, respectively. The analytical curve demonstrated linearity over a concentration range of 0-500 mu mol L-1 with a correlation coefficient (R-2) of 0.9995. The intraday instrumental precision (repeatability) as relative standard deviation (RSD) in terms of peak area and migration time of the 5-HTP were 2.2 and 1.9% (n = 8), respectively. The CE method was applied to the analysis of four different samples of commercial dietary supplements and the RSD (n = 3) for the determinations ranged from 2 to 3%. The accuracy of the method was evaluated comparing the measured 5-HTP concentrations to those obtained by a high performance liquid chromatography (HPLC) method. The relative error ranged from 0 to 5.9% (n = 3) and no significant differences between the concentrations were observed at a confidence level of 95%.254783787Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2008/57805-2, 2008/53868-0, 2012/21787-6]CNPq [305318/2012-8, 476397/2010-3, 573672/2008-3]