Monoamine oxidase catalyzes the oxidative deamination of primary aliphatic and aromatic amines and some secondary and tertiary amines, including the hormone and neurotransmitter amines epinephrine, dopamine, norepinephrine and serotonin. The two isoenzymes, MAO-A and MAO-B, differ according to substrate and inhibitor specificities. For example, selective inhibitors of MAO-A have been shown to be effective antidepressants whereas some MAO-B inhibitors have been reported to be beneficial in the treatment of Parkinson's and perhaps Alzheimer's diseases. Thus, it is important to have accurate procedures for determining the activities of each MAO isoenzyme in tissues that may contain both. In this unit, several MAO assay procedures are described, along with support protocols for equilibrating ion-exchange resin, preparing aldehyde dehydrogenase, preparing dose-response curves for determining the relative proportions of isoenzyme activities, determining concentrations of each isoenzyme, assessing new compounds as putative MAO inhibitors, and determining protein concentration of membrane-bound proteins.