A chromatographic system for the on-line derivatization of drugs using column switching is described. The system uses a 20 mm x 2.1 mm i.d. precolumn packed with a unmodified ODS stationary phase. This column is used for sample cleanup and enrichment of the analytes. Next, the trapped analytes are derivatized by injection of the derivatization reagent into the precolumn. Finally, the derivatives are transferred to the analytical column for their separation under reversed-phase conditions. The influence of several parameters such as the reaction time, the amount of derivatization reagents, or the system design has been studied some amphetamines as model compounds and three derivatization reagents: sodium 1, 2-naphthoquinone-4-sulfonate, o-phthaldialdehyde, and 9-fluorenylmethyl chloroformate. The potential of the described approach is illustrated by determining amphetamine and methamphetamine in untreated urine at ambient temperature.