beta-Carbolines are endogenous inhibitors of monoamine oxidase (MAO). The interaction of nine beta-carboline derivatives and four 3,4-dihydro forms with purified MAO A was investigated. All the compounds tested were reversible competitive inhibitors selective for MAO A, in agreement with previous studies on membrane preparations. The oxidation of kynuramine by MAO A in the presence of the more effective inhibitors showed a lag period before reaching the steady state. In general, the 1-methyl and 7-methoxy substituents increased the potency. Harmine, 2-methylharminium, 2,9-dimethylharminium, and harmaline were the most effective inhibitors of the purified MAO A, with low Ki values of 5, 69, 15, and 48 nM, respectively. The inhibitors interacted with the covalently bound flavin to induce distinct spectral changes, the magnitude of which correlated with the efficacy of the inhibition. The more effective inhibitors could be in situ inhibitors of MAO A.