Diamine oxidase is an enzyme that catalyzes the oxidative deamination of diamines. Putrescine and cadaverine are among the best substrates of porcine kidney diamine oxidase. In this study, a variety of diamines with structures that are related to putrescine and cadaverine, namely N-(3-aminopropyl)aziridine, N-(4-aminobutyl)aziridine, N-(5-aminopentyl)aziridine, N-(6-aminohexyl)aziridine, N-(7-aminoheptyl)aziridine and the rigid analogue, N-(p-aminomethylbenzyl)aziridine have been synthesized and their interactions with diamine oxidase evaluated. These aziridinyl-containing compounds were synthesized in three steps from the condensation of aziridine with bromonitriles to yield aziridinylnitriles. Reduction of the nitriles with LiAlH{dollar}\sb4{dollar} provided the aziridinyl amines. These aziridinyl derivatives were relatively stable in aqueous buffer solutions at 37{dollar}\sp\circ{dollar}C. All of the compounds contain an aziridine ring as an alkylating group to inactivate the enzyme. Calculated IC{dollar}\sb{lcub}50{rcub}{dollar} values ranged from 0.01 to 1.7 mM. The activity of these inhibitors correlated with the separation length of the two nitrogen atoms of the diamine analogues. Four-carbon and five-carbon chain analogues were more potent than the other analogues tested. 3-Aziridinylpropylhydrazine, which contains an aziridine ring as well as a hydrazine group, was synthesized by reduction of the hydrazone formed between 3-aziridinylpropanal and hydrazine. This bifunctional inhibitor showed an IC{dollar}\sb{lcub}50{rcub}{dollar} value of 0.0085 {dollar}\mu{dollar}M which classifies it as one of the most potent inhibitors of diamine oxidase reported. All of the inhibitors showed time-dependent irreversible inhibition of diamine oxidase, which suggests that the formation of an enzyme-inhibitor complex is a reversible step, and a subsequent irreversible step leads to the inactivation of the enzyme. The inhibition pattern depends on the length of the inter-nitrogen distance in the analogues. Evaluation of the kinetics of inhibition of putrescine deamination indicated that the analogues of up to 5 or 6 carbons were competitive inhibitors. The 7-carbon analogue, N-(7-aminoheptyl)aziridine, showed mixed-competitive inhibition and a high IC{dollar}\sb{lcub}50{rcub}{dollar} value. The observed switching of the inhibitory mechanism is consistent with an additional binding region in the active site of diamine oxidase which can be accessed by longer-chained, lower-affinity inhibitors.