Melatonin is a major chronobiological regulator involved in circadian phasing, sleep, and numerous other functions. The novel melatonin agonist Neu-P11 is used in treatment of physiological insomnia. In animal studies Neu-P11 showed sleep-promoting effect. In a phase 1 study Neu-P11 was administered to cohorts of healthy young male volunteers in an ascending single dose study. Up to now no method for measurement of the new drug in human plasma was described and validated. The aim of this study was to develop a suitable analytical procedure. With solid phase extraction (SPE) for sample preparation and liquid chromatography tandem mass spectrometry (LC/MS/MS) a sensitive method with a lower limit of quantification of 0.39. ng/ml was found. For SPE samples were put into Strada C18 cartridges and automatically extracted by a Gilson Automatic Sample Processor ASPEC XL. The HPLC was carried out with a Purospher C18 column and mobile phase gradient with acetonitrile, and ammonium acetate. The precision ranged from 0.4% to 9.5%. The deviation of the measured from the true value, of the standard samples and QCSs was lower than 10%. The method was specific, precise and accurate and the analyte was stable under the conditions of measurement. Results show that the method is suitable for the quantification of NeuP11 in human plasma after a single dose of 5, 20, 50 or 200. mg.