1. The fate ofN-methyl-N′-(hydroxy[14C]methyl)thiourea (MHT) has been studied in the male Sprague—Dawley rat. The compound is degraded toN-methylthiourea and formaldehyde.2.N-Methylthiourea is excreted as a urinary metabolite whereas the formaldehyde is not excreted, either in the urine or the expired air, but is metabolized via formate to CO2.3. At least 50% of an i.p. dose (100 mg/kg) of MHT is excreted unchanged and some of this undergoes hydrolysis within the urine toN-methylthiourea and formaldehyde. Production of formaldehyde leads to the formation of the urinary artefactN-(hydroxymethyl)urea.4. Tissue-distribution studies with14C-MHT have shown that radioactivity is selectively associated with the thyroid gland. A preliminary investigation has indicated that MHT has anti-thyroid hormone activity as it lowers the thyroxine in rat serum.