An assay for nicotinamide methyltransferase which uses an Affi-Gel 601 boronate column to separate substrate from product has been used to study the inhibition of the nicotinamide methylase. S-Adenosylethionine and S-adenosylhomocysteine inhibited the enzyme in a dose dependent fashion. Five times more S-adenosylethionine than S-adenosylhomocysteine was necessary for 50% inhibition of the enzyme. Data from a substrate concentration curve supports a competitive type of inhibition for both inhibitors. It is proposed that this inhibition may alter liver levels of nicotinamide and the urinary excretion of N1-methylnicotinamide.