Abstract A new chiral high-performance liquid chromatographic (HPLC) method utilizing ultraviolet (UV) detection has been developed for determining plasma and urinary concentrations of d-fenfluramine and its major metabolite d-norfenfluramine, while being able to determine the possible presence of l-fenfluramine after oral administration of enantiopure d-fenfluramine hydrochloride. Sensitivity, stability, and specificity were enhanced by derivatizing the extracted analytes with 3,5-dinitrophenylisocyanate while utilizing a Pirkle-type chiral recognition approach. In vitro and in vivo statistical data are analogous. Overall plasma inter-assay precision was less than 7% with a minimum quantitation limit of 10 ng/ml. Overall urine inter-assay precision was also less than 7% with a minimum quantitation limit of 25 ng/ml.