This paper aims a rapid, cheap, simple and modification-free electroanalytical methodology using a boron-doped diamond electrode pretreated anodically for the determination of antithyroid drug methimazole in pharmaceutical samples. The drug was irreversibly oxidized which was a diffusion-controlled process. Using square wave voltammetry, a linear response was obtained for methimazole determination in Britton-Robinson buffer (pH 5.0) at +0.89V (vs. Ag/AgCl). Linearity was found between the oxidation peak currents and the concentration of methimazole from 0.5 to 25.0 mu g mL(-1) (4.38x10(-6)-2.19x10(-4)M) with a detection limit of 0.085 mu g mL(-1) (7.45x10(-7) M). The proposed method was applied for the determination of methimazole in commercial tablet samples.