The anti-inflammatory activity of 1-methylimidazole-2-thiol (methimazole), the most widely used antithyroid drug, was investigated. Methimazole had a marked inhibitory action on prostaglandin H synthase (IC50 = 10 mumol/l), inhibiting the peroxidase (IC50 = 330 mumol/l), although the cyclo-oxygenase was slightly activated. Methimazole was less potent than indometacin (IC50 = 1.7 mumol/l) on prostaglandin H synthase, but was more potent than acetylsalicylic acid (IC50 = 160 mumol/l). Methimazole has been found to trap superoxide (O2.-) radicals and to decrease the level of blood prostaglandin E2.