Monoamine oxidase (E.C.I.4.3.4) (MAO) oxidatively deaminates the biogenic amines normally present in the organism. The activities of the neurons utilizing these amines i.e. noradrenaline, dopamine and serotonin, are supposed to be involved in the pathogenesis of various psychiatric diseases. It is speculated that the MAO activity is changed as well as the monoaminergic activity in some psychiatric disorders. In the present thesis the MAO activity has been studied in brain tissue and in platelets in some psychiatric disorders. The result was as follows: MAO activities in different parts of the human brain seem to be highly intercorrelated in each individual. The brain MAO activity is also weakly correlated both to the concentration of 5-HT and of 5-HIAA, which may indicate that the MAO activity reflects the serotoninergic turnover in the brain. The MAO activity in brains from 15 suicides was compared to a control material of 20 individuals without known mental disorders, and it was found to be lower in the suicides in all 13 analysed brain parts. As eight of the patients had been chronic alcoholics, they were excluded and the remaining seven non-alcoholic suicides were tested as regards MAO activity by analysis of variance and still found to have significantly lower MAO activity than the controls. The eight chronic alcoholics in the suicide series had the most significantly (p<0.005) reduction of the MAO activity as compared to the control group. Rats were given chronic treatments with ethanol, either by 10 °/o ethanol as the only water supply or by exposition to ethanol vapor twice a day. In neither of these cases was the brain MAO activity changed as compared to control rats. The result supports the hypothesis that the low MAO activity found in alcoholic suicides most likely is related to a constitutional factor and not to a direct effect of the ethanol intake. Platelet MAO activity was found to be significantly reduced in human alcoholics as compared to matched controls. If samples were drawn from the alcoholic patients during their abstinence phase, there could be seen a transitory rise in the platelet MAO activity. This increased activity had its maximum after two weeks, and after four weeks the MAO activity had returned to the initial, low level. No difference as regards MAO activity, neither in brain tissue nor in platelets, could be registered when chronic schizophrenics were compared to matched controls. Reduced brain MAO activity was found in a group of patients diagnosed as cycloid psychoses when comparing the activity to controls or to the schizophrenic patients. The platelet MAO activity was also found to be lower in cycloid psychoses than in a group of unipolar affective psychoses, who repeatedly have been found not to differ from normals. These findings suggest that low MAO activities in brain and platelets reflect a phychic constitution in the individual making him more vulnerable for suicidal behaviour, ethanol abuse or cycloid psychosis. / digitalisering@umu.se