Amin oxydase (monoaminoxydase, MAO) is an enzyme which catalyses chemical reactions of biogenic amines. It plays a crucial role in pathogenesis of mental disorders associated with the dysfunction of the central monoaminergic systems (schizophrenia, affective disorders, some forms of alcohol dependence, and personality disorders). MAO has got two isoforms such as MAO-A and MAO-B. The genes coding of MAO are localised at the short arm of chromosome Xp11. In each sequence of genes there is a probability of functional polymorphism occurrence which leads to a variable expression or a change of MAO activity and it exerts an impact on the onset of some mental disorders, such as: schizophrenia, affective disorders, some forms of alcohol dependence, and personality and behavioural disorders. Dynamic development of psychiatric genetics may have crucial impact on considerable progress in understanding molecular background of mental disorders.