A method for the separation and quantitative determination of neuroactive amino acids (aspartate, glutamate, citrulline, arginine, glycine, taurine, gamma-aminobutyric acid) and neuroactive amines (noradrenaline, dopamine and serotonin) in a single chromatographic analysis is presented. The method is based on pre-column derivatization with o-phthalaldehyde and tert.-butyl thiol, on-column preconcentration and separation using 50 microns I.D. packed capillary columns, and detection by amperometry. Mass limits of detection are 80-900 amol for all neurotransmitters with RSDs of 0.71 and 4.6% or better for retention time and peak area, respectively. The method was demonstrated by application to the determination of neurotransmitters in microdialysis samples collected from striatum of live rats and tissue samples extracted from butterfly brains.