Syntheses of a large number of mono- and bicyclic, as well as a few tricyclic, amidine derivatives related to 2,3,4,6,7,8,-hexahydropyrrolo[1,2-a]pyrimidine (DBN) are reported. In vitro potencies for inhibition of the enzyme indolamine N-methyltransferase (INMT) from rabbit and human lung are presented. Four bicyclic amidine derivatives and 11 monocyclic derivatives were found to be equal or superior to DBN in in vitro potencies. With the bicyclic amidines, increasing ring size or introduction of substituents reduced activity. Among the monocyclic analogues, the most potent representatives were five- or six-membered systems with an exocyclic imino group, combined with methyl of ethyl substituents on the endocyclic nitrogen. Introduction of additonal substituents decreased inhibitory potency. 2,3,5,6-Tetrahydro-8H-imidazo[2,1-c][1,4]thiazine and 3-methyl-2-iminothiazolidine have been shown to cause inhibition of lung INMT when administered orally to rabbits.