A reliable reversed-phase high-performance liquid chromatographic method has been developed for the determination of terbinafine (Terb) and its desmethyl metabolite (DMT) in human plasma. The analytes and the internal standard (I.S.) are extracted by a liquid-liquid technique followed by an aqueous back-extraction, allowing injection of an aqueous solvent in the HPLC system. The mobile phase is acetonitrile + 0.012 M triethylamine -0.020 M orthophosphoric acid (50:50, v/v) and the UV detection is at 224 nm. The inter-assay precision over the concentration range 2-1000 ng/ml is between 2.9 and 9.8% for both compounds. The limit of quantification, 2 ng/ml for both compounds, is sufficient for investigating the pharmacokinetics of Lamisil in human studies. With an additional preparation step, this method can be used for assaying Terb in tissues such as nail, sebum and stratum corneum.