A new series of aminopropyltransferase inhibitors has been designed in which the nuclephilic aminopropyl acceptor is attached to the aminopropyl donor, S-adenosyl-1-(methylthio)-3-propylamine (decarboxylated S-adenosylmethionine), to form a "multisubstrate adduct". In the present case, S-adenosyl-1,8-diamino-3-thiooctane (2b) and the corresponding methysulfonium salt (3b) have been synthesized. Several compounds of this type were assayed as inhibitors of spermidine synthase, and both 2b and 3b were found to be potent inhibitors of the enzyme. The thioether 2b is the most potent inhibitor of spermidine synthase described to date and is almost totally devoid of inhibitory activity against the closely related aminopropyltransferase, spermine synthase. This type of compound should have use as a specific inhibitor of spermidine biosynthesis in vivo.