The in vivo formation of dimethyltryptamine was studied in rabbits, rats and monkeys. When C14 labelled N methyltryptamine was administered by intravenous injection to rabbits, C14 dimethyltryptamine was found in lung, the principal site of the methyltransferase that biosynthesizes this psychotogen. Unequivocal evidence for C14 dimethyltryptamine formation in rat tissues was not obtained. When rabbits were given non radioactive N methyltryptamine intravenously, dimethyltryptamine appeared in carotid arterial blood, peaking within the first minute after injection of the precursor. Using this assay procedure dimethyltryptamine synthesis in the rhesus monkey could not be demonstrated.