Dimethyltryptamine has been tested after MAO inhibition (400 mg iproniazid). The well-known aspect of the experimental psychosis could not be observed, there were only few hallucinations. The volunteers had an odd feeling of a changed personality which could be resembled only to the schizophrenic "Wahnstimmung", which precedes the outbreak of the psychosis. These facts seem to support the assumption that there is a serotonin antagonism in the mechanism of action of the hallucinogenic drugs, and on the other hand they draw the attention to a relation between the experimental psychoses and schizophrenia itself, in view of the indole-theory of the latter.