An on-line pre-column derivatization method for the determination of low concentrations of amphetamines using ultraviolet detection has been developed. In this work 9-fluorenylmethyl chloroformate (FMOC-Cl) has been used as the derivatizing agent. The optimum conditions for derivatization such as pH, reaction time and FMOC-Cl/amphetamine concentration ratio have been investigated. Attempts have been made to extend the sensitivity of the method by preconcentration of the derivatives on a micro-column packed with C₁₈ bonded silica. Derivatization and preconcentration of the samples were carried out at low pressure on a flow injection analysis system. Quantitative determination of amphetamines as low as 2 • 10^-8 mol/l can be made using this on-line method with preconcentration. The sensitivity of this technique is about 50 times greater the equivalent off-line method.