It is shown that reversed-flow micellar electrokinetic chromatography in buffer electrolyte solutions with pH < 2.5 can be used to determine catecholamines and their metabolites. The effect of ion-pair (sodium octane sulfonate) and complexing agent (18-crown-6 and 4,13-diaza-18-crown-6) concentrations on the efficiency and selectivity of the separation of biological amines is studied. Complex formation in the macrocycle-analyte system is quantitatively estimated. An on-line version of preconcentration (sweeping) in the presence of 4 mM of 18-crown-6 and 5 mM of cyclohexylamine introduced into the sample matrix is proposed (the preconcentration factor is 900-1500).