The metabolism of [2-14C]methimazole in the rat

1. The metabolism of [2-14C]methimazole was studied in Sprague-Dawley rats after a daily intraperitoneal dose of 17.6 mg/kg for three days. 2. 78.4% of the administered 14C was excreted in the urine; only 6.7% dose as methimazole. After extraction of the urine with chloroform and n-butanol, 47.5% of administered 14C remained in the urine. 3. Six solvent-extractable metabolites were isolated and characterized by t.l.c. and high-resolution mass spectrometry, as N-methylimidazole, S-methylmethimazole, 3-methyl-2-thiohydantoin, 1-methyl-2-thiohydantoic acid, N-methylthiourea and a methylhydantoin.