An improved method for plasma methimazole assay using high performance liquid chromatography is described. The plasma samples were treated with sodium bisulfite and ammonium sulfate prior to extraction with chloroform. This pretreatment of the samples raised the extraction coefficient to 90%, while simple extraction yielded only 55%. The minimal detection limit was 0.02 microgram/ml, and the coefficient of variation at the level of 0.2 and 1.0 microgram/ml was less than 5%. Pharmacokinetics of methimazole was studied after a single oral dose (20 mg/m2) in six subjects including two healthy adults and four thyrotoxic children. Plasma levels of methimazole showed a peak concentration of 1.03 +/- 0.25 microgram/ml approximately one hour after the drug administration. Plasma half-life, area under the curve and distribution volume were 4.56 +/- 0.71 hr, 7.05 +/- 0.95 microgram/ml X hr, and 630 +/- 110 ml/kg respectively.