By using a fluorescent derivatization and column-switching technique, a highly sensitive and selective high-performance liquid chromatographic (HPLC) method has been developed for the determination of simvastatin (I, beta-hydroxy-delta-lactone form) and its active hydrolyzed metabolite (II, beta,delta-dihydroxy acid form of I) in human plasma. A plasma sample spiked with internal standards was applied to a C8 solid-phase extraction column. Compounds I and II were separately extracted from plasma into two fractions. Compound I in one of the fractions was hydrolyzed to II. A fluorescent derivative was prepared by esterification of II with 1-bromoacetylpyrene in the presence of 18-crown-6 for both fractions. The pyrenacyl ester of II thus obtained was purified on a phenylboronic acid (PBA) solid-phase extraction column, and was measured by column-switching HPLC with fluorescence detection. The calibration curves for both I and II were linear in the concentration range of 0.1-10 ng/ml. The intra-day coefficients of variation were less than 11.0%, and the accuracies were between 91.7% and 117% within the concentration range for both analytes. The limits of quantification (LOQ) for both analytes were set to 0.1 ng/ml. This assay method has adequate sensitivity and selectivity to measure the concentrations of I and II in human plasma from clinical studies.