5, 6-Dihydroxyindole (5, 6-DHI) and a series of 4-and/or 7-methylated analogues of 5, 6-DHI have been synthesized and evaluated for their ability to inactivate purified rat liver catechol 0- methyltransferase (COMT). The inactivation of COMT by these agents could be prevented by excluding oxygen from the incubation mixtures, indicating the necessity for their oxidation to the corresponding aminochromes. Substrate protection studies and kinetic studies ...