Catalytic activities of two amine N-methyltransferases were documented for the following azaheterocycles: isomeric phenyl- and bispyridyls; 2-, 3- and 4-mono-substituted pyridines; and a miscellaneous group of azaheterocycles that included mono- and diazabenzenes and mono- and diazanaphthalenes. The broad substrate specificities of the two amine N-methyltransferases for primary and secondary amines are here extended to a large number of aromatic azaheterocycles in which N-methylation results in the formation of quaternary ammonium metabolites. Pyridine was the best substrate for both enzymes. Substitution in the ring at the 2-position sterically hindered methylation of the pyridyl nitrogen; 2-phenylpyridine and 2,2'-bispyridyl were not substrates.