A sensitive method for spectrophotometric determination of methimazole is based on the reaction of this compound with 2,3-dichloro-1,4-naphtoquinone in presence of ammonia. The colour thus formed is stable, reproducible and linear in the range 0,4 to 4 micrograms/ml of methimazole concentration. This method has been employed successfully for estimation of the antithyroid drug in plasma sample with good recovery (75 +/- 2%). The colour is due to formation of a red compound of molecular formula C14H11O2N3S, deduced from elemental analysis and mass spectrum.