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The detection limit of tramadol hydrochloride was effectively improved by proper selection of a plasticizer and ion exchanger. Two plastic membrane electrodes for the determination of tramadol hydrochloride (TDCl) were fabricated and fully characterized in terms of composition, life span, usable pH range and working concentration range. The membranes of these electrodes consist of an ion-exchanger such as tramadolium-silicotungstate (TD-ST), silicomolybdate (TD-SM), dispersed in PVC matrix with different plasticizers, namely 2-nitrophenyl octyl ether (2-NPOE), dioctyl phthalate (DOP), dibutyl phthalate (DBP), tris(2-ethylhexyl) phosphate (TEPh), dioctyl sebacate (DOS), tributyl phosphate (TBPh) and dibutyl butyl phosphonate (DBBPh). Experiments showed that DBP was the best plasticizer and TD-PT The best ion pair for their combination produced the lowest detection limit. The present electrodes show clear discrimination of tramadol hydrochloride from several inorganic, organic ions, sugars and some common drug excipients. The sensors were applied for determination of tramadol hydrochloride in urine, milk and pharmaceutical preparations using potentiometric determination, standard addition and the calibration curve methods. The results obtained were satisfactory with excellent percentage recovery comparable and sometimes better than those obtained by other routine methods for the assay. [ABSTRACT FROM AUTHOR]