Structural analogues of S-adenosyl-L-methionine (SAM), with modifications in the amino acid, sugar, or base portions of the molecule, have been synthesized and evaluated as either inhibitors and/or substrates for the enzymes catechol O-methyltransferase, phenylethanolamine N-methyltransferase, histamine N-methyltransferase, and hydroxyindole O-methyltransferase. To evaluate these analogues as substrates for SAM-dependent methyltransferases, the corresponding methyl-14C compounds were prepared and tested for their abilities to donate their methyl group to the appropriate acceptor molecules. In addition, the unlabeled SAM analogues were tested for their inhibitory activities in these same transmethylation reactions. In general, it could be conlcuded from these studies that methyltransferases show very strict specificity for the structural features of SAM. This strict specificity holds for the enzymatic binding and methyl-donating abilities of this molecule. In fact, it could be concluded from the results of this study that methyltransferases show a higher specificity for the structural features of the substrate L-SAM than for the structural features of the product S-adenosyl-L-homocysteine (L-SAH).