This study describes the optimal conditions for the determination of Thioridazine HCl (Tz-HCl) in 0.2 M sulphuric acid by voltammetry using ruthenium (Ru), platinum (Pt) and glassy carbon (GC) electrodes. A new and sensitive method is described for electrochemical determination of Tz-HCl in the concentration range 5.10(-5) - 10(-3) M (20.35-407 mu gml(-1)); 6.10(-4) - 10(-2) M (244.2 - 4070 mu gml(-1)); 10(-4) - 10(-3) M (40,7 - 407 mu gml(-1)), with Ru, Pt and glassy carbon electrodes, respectively. The method was applied successfully to the determination of Tz-HCl in pure form or incorporated in their representative pharmaceutical preparations. The precision of the assay was comparable with that of the official assays.