In establishing the role of inherited variations in levels of monoamine oxidase (MAO) activity in neuropsychiatric diseases, it is important to measure levels of both A and B types of activity as they appear to be under separate genetic control. Levels of A and B types of activity can be evaluated in fibroblasts and platelets, respectively. A number of genes could be involved in determining levels of activity, including those coding for the catalytic and noncatalytic subunits of the enzyme, as well as those coding for enzymes involved in covalent attachment of the flavin cofactor, other processing steps, degradation of MAO, and lipid metabolism. Different genes may be critical in controlling activity levels in various cell types depending on differential expression of the genome. In order to establish the molecular basis of variation in activity, techniques should be employed to assess the structure and conformation of the enzyme, as well as the number of enzyme molecules and their interaction with other cellular components. Only by understanding the genetic and environmental factors controlling levels of A and B types of MAO activity can we hope to evaluate and manipulate the role of MAO in human neurophysiology.