A sensitive and specific method for the determination of the diuretic Ethyl (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene) acetate (etozolin, Elkapin) and its pharmacologically active main metabolite, (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene) acetic acid (ozolinone), at therapeutic concentrations in plasma is described. The method is based on high performance liquid chromatography and the use of two structurally similar internal standards. Etozolin and its metabolite are extracted from the plasma into dichloromethane at pH 9 and pH 5, respectively, and the resulting residues are analyzed on a silica gel column. The elution peaks are detected by UV absorption at 281 nm. The sensitivity for both compounds is 20 ng/ml plasma. The precision of the method is about +/- 5%. The applicability to pharmacokinetic studies of etozolin is shown.