The study of biogenic amines, which from the beginning of modern pharma- cology has always attracted the enthusiastic attention of research workers and has led to some of the most brilliant and illuminating experimental results in biology, has been unexpectedly extended in the last fifteen years. 'With the discovery of 5. hydroxytryptamine (enteramine, serotonin), in fact, the vast field of indole- alkylamines has been opened up for research. In a decade of intensive and dedicated work, an immense amount of experi- mental data has been collected, the significance of which, having rapidly overrun the limits of pharmacology, physiology, and biochemistry, has spread into all fields of biology and medicine, as previously occurred with choline esters, catechol- amines and imidazolealkylamines. Thus, the study of indolealkylamines, espe- cially that of their metabolism, has led not only to explanation of the mechanism of action, formerly largely obscure, of some important drugs, to clarification of the pathogenesis of some morbid syndromes and to a parallel flourishing of research on other biogenic amines, but also to surprising achievements in the field of therapeutics. The discovery of monoamineoxidase and dopadecarboxylase inhibitors, and of drugs which block the storage of biogenic amines in the body depots are among the most striking results of research on the indolealkylamines.