The behavioral action and the tremor-producing activity and metabolism of 5-methoxy-dimethyltryptamine, dimethyltryptamine, and bufotenine has been studied. 5-Methoxydimethyltryptamine in low doses elicits hyperactivity in a conditioned avoidance response situation. This effect is enhanced by pretreatment with SKF 525 A. 5-Methoxydimethyltryptamine and, to a lesser degree, dimethyltryptamine and bufotenine give rise to tremor. The tremor is antagonized by pretreatment with chlorpromazine and enhanced by pretreatment with SKF 525 A and an MAO inhibitor. Atropine does not counteract the tremor. After administration of radioactive 5-methoxy-dimethyltryptamine 60% of the radioactivity was excreted within 12 hours. The metabolic fate of the three substances has been studied in rats, and metabolites in urine identified by gas chromatography and mass spectrometry. The main metabolite of administered 5-methoxy-dimethyltryptamine is 5-methoxyindoleacetic acid which represents a major part of the urinary excretion. Of the other compounds, the corresponding acids seem to represent main excretion products. The metabolism of the indolealkylamines and the theory of 6-hydroxylation are discussed.