Multisubstrate adducts of the indole N-methyltransferase reaction have been designed in which a structural moiety representing the nucleophilic methyl acceptor is attached through the sulfur atom to the 5-(methylthio)adenosine and/or methionine moieties of the methyl donor, S-adenosyl-L-methionine. Indole derivatives attached through a 4-(3-indolyl)butyl sulfide or a 3-(3-indolyl)propyl sulfide linkage to 5'-thioadenosine or homocysteine have been synthesized, together with their corresponding methylsulfonium salts. These compounds have been assayed for their ability to inhibit rabbit lung indole N-methyltransferase. The adenosylsulfonium salts (5'-deoxyadenosyl)[4-(3-indolyl)but-1-yl]methylsulfonium perchlorate and (5'-deoxyadenosyl)[3-(3-indolyl)prop-1-yl]-methylsulfonium perchlorate were found to be inhibitors of this enzyme with Ki's of 12 and 44 microM, respectively. Neither of these compounds was effective in inhibiting the methylation of 3,4-dihydroxybenzoic acid, catalyzed by purified porcine catechol O-methyltransferase.