A high-performance liquid chromatographic method for an effective determination of glafenine and its main metabolite, glafenic acid, is described. The assay involves separate extraction procedures for glafenine and for its metabolite, but the same internal standard (floctafenine) and the same chromatographic conditions (including a 5-micron C8 column, a quaternary solvent mixture of water-acetonitrile-diethylamine-acetic acid and an ultraviolet detector set at 360 nm). For 1 ml of plasma, the detection limit is 0.05 mg/l for glafenine and 0.25 mg/l for glafenic acid. Compared with previously described techniques, this assay uses a very low glafenine linearity range, which allows the true pharmacokinetics of this drug to be described for the first time.