1 Carbimazole was administered to nine hyperthyroid patients, and blood samples were taken at various time intervals for analysis of carbimazole, and its metabolite methimazole.2 A technique was developed for the measurement of methimazole in serum using a High-Pressure Liquid Chromatograph, which could detect nanogram quantities of this metabolite.3 After a single oral dose, the patients' blood levels appeared to fall into two groups, either those with a maximum concentration of methimazole between 30 and 60 min, or those whose maximum was 2 to 3 hours.4 The results would suggest that there could be a correlation between the high level of methimazole in serum and the high thyroxine concentration found in some patients.