In this study, a rapid and selective method using microdialysis liquid chromatography/tandem mass spectrometry was developed for the in vivo analysis of the catecholamine dopamine, and its metabolites DOPAC and HVA, and the indolamine serotonin, and its metabolite 5-HIAA.  Sampling of the extracellular fluid by microdialysis enables continuous monitoring of intercellular chemical concentrations, which allows for the study of neuronal communications. Liquid chromatography using a Fluophase PFP column (1.0 x 50 mm, 5 om), packed with a fluorinated phase, provided sufficient retention and separation of both monoamine neurotransmitters and their respective acid metabolites. This analytical column allowed for online desalting, which was critical for successful detection, by combining hydrophilic interaction chromatography (HILIC) and reverse phase chromatography. Mass spectrometric detection was performed using electrospray ionization coupled to an AB/MDS Sciex API3000 triple quadrupole mass spectrometer operating in both negative and positive ion modes. (Abstract shortened by UMI.)