Structural analogues of S-adenosyl-L-homocysteine (L-SAH), with modifications in the amino acid or base portions of the molecule, have been synthesized and evaluated for their abilities to inhibit the transmethylations catalyzed by catechol O-methyltransferase (COMT), phenylethanolamine N-methyltransferase (PNMT), histamine N-methyltransferase (HMT), hydroxyindole O-methyltransferase (HIOMT), and indoleethylamine N-methyltransferase (INMT). From these studies some interesting and potentially useful differences in the structural features of L-SAH needed to produce maximal binding to these methyltransferases were detected. This paper provides evidence that 8-azaadenosyl-L-homocysteine is a potent and selective inhibitor of HIOMT, whereas N6-methyladenosyl-L-homocysteine and N6-methyl-3-deazaadenosyl-L-homocysteine are selective inhibitors in INMT. In contrast, it was found that S-tubercidinyl-L-homocysteine was a fairly potent, but nonselective inhibitor of all of the methyltransferases studied. The differences and the similarities in the requirements for the binding of SAH to methyltransferases which were detected in this study and earlier studies from our laboratory, are described. The possibilites of utilizing differences in binding requirements for the design of SAH analogues as specific inhibitors of methyltransferases are discussed.