The electrochemical oxidation of melatonin is studied in Britton-Robinson buffer in the pH range 2.2-11.8 by cyclic voltammetry at glassy carbon electrode. Three irreversible, diffusion-controlled oxidation peaks were obtained. The utility of using differential pulse and square wave techniques for de-termination of melatonin was examined. Different parameters affecting the peak current were analylzed and at optimum conditions, linear calibration plots from 0.02-0.5 mM and 0.04 to 0.5 mM of melatonin were obtained applying the two techniques, respectively. The differential pulse mode was successfully ap-plied to the determination of melatonin in commercial tablets containing about three-folds of vitamin B6 without previous separation. The same technique was also used for melatonin determination in human serum with acceptable accuracy. INTRODUCTION Melatonin (Fig. 1), [N-[2-(5-methoxy-1H-in-dol-3-yl)ethyl] acetamide; N-acetyl 5-methoxy triptamine], is a natural hormone made by pineal gland, which lies at the base of the brain. Its secretion is stimulated by the dark and inhib-ited by light. Melatonin is a highly important an-tioxidant; protect cells from free-radical damage. This hormone is now a popular therapy for jet-lag and disturbances of sleep 1 . Although mela-tonin has extensively been detected in the ani-mal kingdom, recently this compound has also been found in different structures of higher plants like leaves, fruits and seeds. The levels are too low, however, to provide a significant melatonin supply 2 . Several analytical methods have been reported for determination of mela-tonin, such as UV spectrophotometry in solid dosage forms 3 , UV/visible spectrophotometry 4 , colorimetry 5 , chemilumenescence 6 , gas chro-matography-mass spectrometry 7 , high-perfor-mance liquid chromatography 8 and thin-layer chromatography 9 . Melatonin has also been de-termined in biological samples by capillary elec-trophoresis 10 . A radioimmunoassay method of bound and free melatonin was applied to hu-man plasma 11 . Melatonin is an electroactive substance that can be easily oxidized and is, thus, suitable for analyses with electrochemical methods. In re-cent years, there has been an increased interest in developing accurate analytical methods valid for quantitative determination of melatonin. A sensitive amperometric method was described for detection of melatonin in rat pineal gland and tablets 12 , using a carbon fiber micro disk electrode as a detector at the end of a capillary electrophoresis column. Another amperometric method was described using a carbon disk elec-trode for determination of melatonin after sepa-ration from pyridoxine in pharmaceutical