Analogues of S-adenosyl-L-methionine (1) were synthesized and evaluated as inhibitors of the enzyme S-adenosyl-L-methionine decarboxylase from rat liver. The compounds synthesized were S-(5'-deoxy-5'-adenosyl)-(±)-2-methylhomocysteine (10), S-(5'-deoxy-5'-adenosyl)-(±)-2-methylmethionine dihydrogen sulfate (11), S-(5'-deoxy-5'-adenosyl)-(±)-2-methylhomocysteine sulfoxide (12), S-(5'-deoxy-5'-adenosyl)-(±)-1-methyl-3-thiopropylamine hydrogen sulfate (16), S-(5'-deoxy-5'-adenosyl)-(±)-1-methyl-3-(methylthio)propylamine dihydrogen sulfate (17), S-(5'-deoxy-5'-adenosyl)-(±)-1-methyl-3-thiopropylamine sulfoxide hydrogen sulfate (18), N-(cyanoethyl)-N-methyl-5'-amino-5'-deoxyadenosine (20), and N-(aminopropyl)-N-methyl-5'-amino-5'-deoxyadenosine dihydrochloride (21). S-Adenosyl-L-methionine decarboxylase was partially purified from rat liver homogenate using a methylglyoxal bis(guanylhydrazone) linked Sepharose column in the final purification step. At the highest concentration used (2.5 × 10^-4 M), compounds 10, 12, 16, 18, and 20 did not produce inhibition of the enzymatic decarboxylation of [14C]carboxyl-labeled S-adenosyl-L-methionine. Compounds 11, 17, and 21 were competitive inhibitors of S-adenosyl-L-methionine decarboxylase, and the apparent Ki values for these compounds were calculated to be 1.8 × 10^-4, 1.2 × 10^-4, and 1.1 × 10^-4 M, respectively. Compounds 11 and 17 formed azomethine bonds with an essential carbonyl group in the enzyme active site which was reducible with sodium cyanoborohydride. These two inhibitors also caused a time-dependent inactivation of the enzyme, which was also dependent on the concentration of the inhibitor in the incubation media. Compound 21 did not form an azomethine bond with the enzyme and did not cause inactivation of the enzyme. These results suggest that the sulfonium analogues 11 and 17 have a binding mode to the enzyme active site which is different than that for the nitrogen analogue 21.