The pharmacokinetics (serum, urinary and biliary concentrations) of Isotiod have been studied in rats. Doses exploited the property of the molecule in an alkaline medium with a diazonium salt to form coloured azo compounds. In rats Isotiod determines blood concentrations whose intensity and duration are proportional to the dose and route of administration; notable quantities of the drug are eliminated by the urinary way and a small quantity by the bile way with a personal technique.