Beside a large number of pharmaceuticals available on our market having an impact on serotonergic transmission, selective serotonin reuptake blocker inhibitors (fluvoxamine, fluoxetine) and a selective reversible monoaminooxidase inhibitor (moclobemide), have been included recently. The combination of the drugs can in certain conditions cause a central serotonergic hyperactivity, i.e. the serotonin syndrome. The study describes the development of the serotonin syndrome following an abrupt replacement of trazodone by moclobemide. The diagnosis was made on the basis of the anamnesis, clinical features, course and outcome of the disorder and diagnostic criteria suggested in the literature. The importance of the prevention, early detection and timely management of the serotonin syndrome have been reflected in the fact that some of the outcomes described were fatal, although this state most frequently remits spontaneously after the replacement of the drug therapy.